Meteor scatter burst communication system

ABSTRACT

The specfication discloses a meteor burst communication system including at least two spaced apart master stations, in association with a plurality of groups of remote stations spaced at locations remote from the master stations, with each group being associated with one of the master stations. Each of the master stations includes a radio transmitter for transmitting probing digital radio signals having address portions which are directed from the master station for reflection from meteor trails to the remote stations associated with the master station. Circuitry is provided to vary the length of the address portions transmitted by the master station in dependence upon desired signal-to-noise and timing considerations. Each of the remote stations includes a radio receiver for receiving the reflected probing digital radio signals from the master stations. Each of the remote stations also includes at least one sensor of physical characteristics such as snow depth, rain fall or the like. The remote stations further include a predetermined unique digital address sequence stored therein and include circuitry to compare the received address portions from the master station with the stored digital address sequence. The remote stations each include a transmitter for transmitting digital data representative of the output of the associated sensor to the master station via reflection from a meteor vapor trail, if the received address portion compares with the stored digital address sequence in accordance with predetermined criteria.

FIELD OF THE INVENTION

This invention relates to digital communication systems, and moreparticularly relates to a meteor scatter burst communications systemutilizing a plurality of remote data terminals.

THE PRIOR ART

In numerous applications, it is desirable to transmit information from amaster station to one or more remote stations over such long distancesthat it is not possible to interconnect the master station with theremote stations by wires and wherein it is not practical to useconventional radio communication systems. It has been previously knownto conduct such remote data transmission utilizing orbiting satellitestations. However, the use of such satellite stations is expensive andis often impractical due to the limited number of such satellitestations available.

It has thus also been previously known to utilize meteor scattertechniques, commonly termed meteor burst communications, in order toenable radio communication over long distances. Such meteor burstcommunication utilizes the generation of radio waves in the low VHFfrequency range from a master station and includes the reflection ofsuch radio waves from electrons in meteor trails. The reflected radiowaves may then be detected by remote stations for distances greater than1000 miles. Inasmuch as such meteor trails usually exist only for a fewmilliseconds to a few seconds, a burst transmission mode is commonlyused in such techniques. Because the availability of properly oriented,suitably ionized meteor trails varies with the time of day and the monthof the year, such systems require the proper selection of operatingfrequencies and design parameters for consistent practical performance.

Numerous governmental agencies have developed and experimented withmeteor burst communications for the past 25 years. In the 1960's, a twoway experimental meteor burst communications link was developed byMontana State University for the Bonneville Power Administration. TheBonneville Power Administration system demonstrated the practicalcapability of meteor bursts in the gathering of hydrometeoroligical datafrom a limited number of remote stations. Further, in 1974 and 1975,federal agencies in Alaska demonstrated the use of meteor burstcommunications to collect remote data in Alaska involving several remotedata terminals.

While such prior developmental programs have indicated the generalpracticality of meteor burst communication systems, such prior systemshave not utilized a large number of remote stations and have thereforenot solved the many problems inherent in the selective polling ofdesired remote stations from as many as several hundred remote stations.Moreover, such prior systems have not been capable of obtaining a highresolution of data from a large number of remote stations whilemaintaining consistency and accuracy of data selection and have furthernot provided substantial flexibility in the selection of or polling ofremote stations in order to obtain selected data. A need has thus arisenfor a meteor burst communications system which enables continuousautomatic collection of accurate sensor data from a large number ofremote data collection stations. Such a system must be flexible toenable easy addition or elimination of remote station data collectionpoints, and must include techniques to insure the accuracy andconsistency of the collected data in varying transmission conditions. Inparticular, a need has arisen for a meteor burst communication systemwhich may be utilized to collect weather data, such as temperature, snowdepth, precipitation and river depth data, from a plurality of remoteand relatively inaccessible remote stations on a continual basis inorder to assist in various weather projections.

SUMMARY OF THE INVENTION

In accordance with the present invention, a meteor burst communicationsystem is provided which enables automatic and accurate communicationswith any selected one or a group of a large number of remote stationsunder varying transmission conditions.

In accordance with the present invention, a system for meteor burstcommunication include circuitry for generating polling or probing codesfrom a master station. Reflections of the polling code from meteortrails are received at a multiplicity of remote stations. Responsesignal are transmitted from each of the remote stations having anaddress included in the polling codes. Circuitry is provided to vary thelength of the polling code transmitted by the master station independence upon the remote stations generating response signals.

In accordance with another aspect of the invention, a meteor burstcommunication system includes at least one master station and aplurality of remote stations spaced at locations from the masterstation. The master station includes a radio transmitter fortransmitting probing digital radio signals having address portions ofselectively variable length. The probing digital radio signals aredirected from the master station for reflection from meteor vapor trailsto the remote stations. Each of the remote stations include a radioreceiver for receiving the reflected probing digital radio signals fromthe master station. Each of the remote stations includes addressrecognition circuitry having a predetermined digital address sequencestored therein and further having circuitry for comparing the receivedaddress portions with the stored digital address sequence. At least onesensor physical characteristic is connected with each of the remotestations. Each remote station includes a transmitter for transmittingdigital data representative of the output of the sensor to the masterstation via reflection from a meteor vapor trail, if the receivedaddress portion compares with the stored digital address sequence inaccordance with predetermined criteria based upon the length of thereceived address portion.

In accordance with another aspect of the invention, a meteor burstcommunication system includes a master station which intermittentlycontacts a plurality of remote stations. A digital probing radio signalis transmitted from the master station having an address field of n-bitcapacity. A digital address is computed which has a number of bits up ton-bits dependent upon a desired number or remote stations to becontacted. Circuitry applies the digital address for transmission in theaddress field.

In accordance with yet another aspect of the present invention, a meteorburst communication system includes at least one master station and aplurality of remote stations based at locations from the master station.The master station includes a radio transmitter for transmitting probingdigital radio signals. The digital radio signals are directed from themaster station for reflection from meteor trails to the remote stations.Each of the remote stations includes a radio receiver for receiving thereflected probing radio signals from the master station. At least onesensor physical characteristic is connected with each remote station.Each remote station includes a transmitter for transmitting, at aspecified frequency, digital data representative of the output of thesensor to the master station via reflection from a meteor trail. Themaster station includes a plurality of antennas connected to discreteradio receiving channels for simultaneously receiving digital data froma plurality of the remote stations. Circuitry is connected to the radioreceiving channels for storing the received digital data for subsequentuse.

In accordance with yet another aspect of the present invention, a meteorburst communication system includes at least two spaced apart masterstations. A plurality of groups of remote stations are spaced atlocations remote from the master stations and each group is associatedwith one of the master stations. Each of the master stations includes aradio transmitter for transmitting probing digital signals havingaddress portions for being directed from the master station forreflection of meteor trails to the remote stations associated therewith.Each of the remote stations includes a radio receiver for receiving thereflected probing digital radio signals from the master stations. Atleast one sensor of physical characteristics is connected with eachremote station. Each remote station includes a transmitter fortransmitting digital data representative of the output of the sensor tothe master station via reflection from a meteor trail. Circuitry isprovided to detect when one master station receives a response from aremote station associated with another master station and for notifyingthe other master station to eliminate the requirement of further probingfor the received remote station.

DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the invention may be had by reference tothe accompanying drawings, illustrating a preferred embodiment of theinvention to be described in detail, wherein:

FIG. 1 is a general system diagram of the meteor burst communicationsystem of the present invention;

FIG. 2 is a diagrammatic illustration of meteor burst communication;

FIG. 3 is a pictorial diagram of a representative master stationinstallation;

FIG. 4 is a pictorial diagram of a representative remote stationinstallation;

FIG. 5 is a block diagram of the master station hardware;

FIG. 6 is a block diagram of remote station hardware;

FIG. 7 is a block diagram of a portable communication and field testunit;

FIGS. 8-10 are schematic diagrams of circuitry for the master stationcontrol logic portion of the block diagram shown in FIG. 5;

FIGS. 11-15 are schematic diagrams of circuitry for the remote stationcontrol logic portion of the block diagram shown in FIG. 6;

FIGS. 16-18 are outlines of the formats for communication messagesbetween a master station and remote stations;

FIG. 19 is an outline remote station address structure;

FIG. 20 is a diagram of formats for communication messages between amaster station and a portable communications and field test unit; and

FIGS. 21-27 are flow diagrams illustrating subroutines relating to thegeneration by the digital computer of polling sequences and probes.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIG. 1, a somewhat diagrammatic block diagram is providedof the preferred embodiment of the present invention. A central servicecenter is designated by the numeral 10 and serves to coordinate theoperation of a master station A identified by the numeral 12 and amaster station B identified by the numeral 14. Master stations A and Bwill generally be separated by relatively large distances in order toincrease the area which is able to be covered by the present system. Ina specific implementation of the present invention, the master station Awas located in the vicinity of Ogden, Utah, while the master station Bwas situated in the vicinity of Boise, Idaho.

Each of the master stations 12 and 14 is generally responsible forpolling and receiving information from a plurality of remote stations16a-d which are located from a few miles from the master station to amaximum of approximately 800 miles from the master station. It will beunderstood that although four remote stations 16a-d are illustrated inFIG. 1, that in actual implementation of the invention large numbers ofremote stations will generally be utilized, such as several hundredremote stations. Each of the remote stations includes one or moresensors which inputs data regarding physical characteristics such asprecipitation level, temperature and the like. The remote stations, whenproperly addressed, transmit this data to a master station. The data maythen be used to predict such important data such as the effects ofprecipitation and snowfall at remote sites on future water availabilityat other key areas. The remote stations are normally situated andorganized into polling or probing groups within a circumference of 15 to30 miles, for example. Each polling group is then addressed by the samemeteor burst and each polling group is then sequentially addressed fromthe master station. This sequential polling or addressing of pollinggroups operates to prevent more than one site from respondingsimultaneously to the master station on the same meteor trail.

In addition, the invention contemplates the use of one or a plurality ofportable field test units 18a-b. These portable field test units areadapted to be movable by a single operator into the remote areas whereinthe remote station 16a-d are located. The portable field test units18a-b contain circuitry for accomplishing diagnostic testing of theremote stations. In addition, as will be subsequently described, theportable field test units 18a-b include circuitry for enabling theoperator to communicate via the system to either of the master stations12 or 14.

The service center 10 operates to initiate the flow of information ofthe system by requesting a nominal, an ad-hoc or background poll orprobe from both of the master stations 12 and 14. A nominal pollincludes probing all remote stations associated with a master station.An ad-hoc poll enables selective communication with any one or group ofremote stations. A background poll is a general poll for communicationwith, for example, portable field test units. The master stations entera predetermined probing schedule under software control for a fixed timeperiod. At the remote stations 16a-d, data from a plurality of sensorsgenerates new data and the new data is updated in a storage during apredetermined interval such as fifteen minutes and is stored in bufferstorage. When the master station 12 or 14 generates a probing signalwhich is reflected from a meteor burst and is received by the remotestation 16a-d, the transmitter of the remote station is turned on anddigital data is sent via meteor reflection to the master station. If themaster station receives the remote station's transmission and detects noerrors in the message thereof, an acknowledge code is sent to thatspecific remote station. The remote station will then be inhibited fromtransmitting another response for a predetermined period of time or whena special command is transmitted by the master station.

Each of the master stations 12 and 14 includes a digital computer forgenerating the holding signals and for receiving and storing the datafrom the remote station 16a-d. In addition, the master stations maycommunicate with the portable field test units 18a-b and conversely theportable field test units 18a-b may transmit signals to the masterstations. The computers of each master station are linked together by acommunication channel so that the memories in each computer canrecognize receipt of data from remote stations and can speed up thepolling sequence. At the completion of a predetermined polling time, thereceived data from each master station is then transmitted to theservice center 10 for further processing.

One advantage of the present system is its ability to poll or probemultiple remote stations at the same time. A digital signal burstgenerated by one of the master stations and reflected by a meteor trailprojects only a small beam toward the remote station cites, thusproviding a private data link. Therefore, different remote stationsrespond to different meteor trail reflections. If two remote stationsare close in proximity, both could respond and therefore interfere withone another. The required separation between the remote stations 16a-ddepends on the range to the closest master station and generally variesbetween 15 and 30 miles. Remote stations in close proximity to oneanother are generally assigned different polling address codes or areassigned to different master stations.

In the case of a nominal polling technique, each of the master stations12 and 14 begins a predetermined probe sequence designed to interrogateall of the remote stations 16a-d in a timely manner. The remote stationsare normally divided into polling groups and sequences to bestaccommodate the fastest response times. As the remote stations beingpolled respond, their data is stored in the master station computers andthe remote status table in the computer is updated to indicate that theremote stations data has been collected. When all remote stations in agroup have responded or the time allocated for polling a group expires,the probe sequence advances to the next group and polls the remotestations in that group. This polling continues until the pollingsequence is complete, at which time the data collected is reported byboth master stations by way of the communications link to a mastercomputer at the service center 10.

An ad-hoc polling command for the service center 10 is a request toobtain data from specific remote stations. An important aspect of thepresent invention is the ability to select polling of any specificremote station or any group thereof. The way the master stations respondto the request from the service center 10 is a function of the systemresponse time at the time of the request. During morning, year around,and evening hours of summer months, response times will normally besmall enough to allow unique polling of the remote stations requested,so that the data collected may be transmitted to the service center 10.During evening hours of the winter months response times will normallybe sufficiently increased to prevent gathering of ad-hoc data in atimely manner by unique remote polling. Therefore, during the eveninghours of the winter months, the requested remote stations will be polledas part of larger group polls, therefore allowing several of the ad-hocremote stations to be polled simultaneously, while also however pollingmany remote stations not requested in the ad-hoc polling. This willcause some remote stations to transmit more often than necessary, butwill not degrade the overall system's performance. The master stationswill keep track of the remote stations received and when the ad-hocrequest is satisfied, will report the received data to the servicecenter 10.

An important aspect of the invention is the automatic optimizing of theprobing signals transmitted by the master stations. Each master stationhas an address field of ten bits in the probing signal. As the size of agroup of remote stations to be polled increases, the number of bitstransmitted in the address field decreases. Similarly, to customize aprobing signal to one or a few remote stations, the number of bits usedin the address field is increased. Aw will be subsequently be describedin greater detail, the present system automatically adds or subtractsbits from the transmitted address field in order to optimize the probingsignals relative to desired signal-to-noise and timing requirements.

The portable field test units 18a or 18b are capable of transmitting a16 character alphanumeric message to either of the master stations 12 or14 whenever the portable field test unit acquires the master pollingsignal. Likewise, any master station is capable of receiving andprocessing messages from either of the portable field test units 18a or18b. Received messages may either be displayed to personnel at themaster station or forwarded to the service center 10. Conversely, theservice center 10 or the master station personnel can generate 16character messages for transmission to a specific portable field testunit 18a or 18b. These messages may either be transmitted uniquely orapplied via nominal, ad-hoc background polling sequences.

FIG. 2 illustrates somewhat diagrammatically the basic operation of thepresent meteor burst communication system. The master station 12generates a PSK coded signal which requests a specific or group of datato respond, along with a coded message containing data. As illustratedin FIG. 2, this PSK coded signal is transmitted from an antenna 20 onthe master station 12 upwardly over a predetermined angle. Thetransmitted signals are reflected downwardly by meteor vapor trails inthe known manner. For example, FIG. 2 illustrates a meteor region inwhich meteor 22 and 24 occur at the time of transmission of a signalfrom the master station 12. Only signals generally following signal path26 are reflected from the trail of the meteor 22. These signals arereflected generally along the path 28 and are received by the remotestation 16a. Due to the relatively narrow angle of reflection, remotestation 16b does not detect reflections from the trail of meteor 22.However, radio signals from the master station 12 following the generalline 30 are reflected from the trail of meteor 24. These reflectionsfollow the general path 32 and are detected by the remote station 16b.

Thus, as seen in FIG. 2, different remote stations respond to differentmeteor trails, thereby providing relatively private links. In order toprevent interference with such private links, the remote stations aregenerally separated by 15 to 30 miles. Remote stations in very closeproximity will generally be assigned different polling address codes orassigned to different master stations.

The remote stations 16a and 16b are normally dormant with only theirradio receivers activated. Whenever a properly addressed masterstation's polling signal is detected at the remote station receiver, andif the remote station is instructed to send data by the polling signal,the remote station's transmitter is switched on and data is sent.

Vertical horizon clearances at the master and remote stations arerelatively critical since communications between the stations depends onionized meteor trails, with the radio signals being either reflected orreradiated off the meteor trail most often at an obtuse angle from theoriginating master station. The greater the distance between the masterand remote stations, the lower the horizontal obstruction must be toallow both the master and the remote stations to see the meteor trail.The present meteor burst communication system takes into account thetechnical characterisitics and statistical nature of meteor burstphenomena. These characteristics include time intervals betweensuccessive meteor burst reflections and the effect of diurnal andseasonal variations. Other characteristics are the density and thelength of the meteor trails and the effect thereof on radiotransmission, such as side angle reflections which often tend to create"hot spots" outside of a normal antenna pattern.

FIG. 3 illustrates a typical master station installation. The presentsystem is generally enclosed within a shelter 34 to provide protectionfrom the elements. The basic components of the system within the shelter34 comprise a housing 36 which includes a transceiver, a control panel,a digital minicomputer, and power supplies. A second housing 38 includesan additional power amplifier with associated controls and meters. A setof duplex filters 40 comprise two free standing 2×2×7' duplx filters inaddition to two wall-mounted helical resonators.

Four coaxial cables 42a-d interconnect the transceiver of the systemwith four directional antennas 44a-d. Although it will be understoodthat various types of antennas may be utilized, in the preferredembodiment four dual beam five element directional Yagi antennas arehorizontally polarized at each master station. Each antenna is 15 dBforward gain and is mounted atop an 80 foot high guide tower and thetowers are spaced from the shelter 34 at the centers of a square havingsides approximately 40 feet long.

The output of transceiver exciter at each master station is amplified to2,000 watts at 40.53 MHz by the final power amplifier which comprises amodified Henry radio 4K ultra linear amplifer. The duplexer cavities andhelical resonators 40 allow simultaneous operation of the master stationtransmitter and receivers each with separate frequencies utilizing thesame for antennas 44a-d. In this manner, data may be simultaneouslyreceived and buffed from a plurality of different remote stations.

FIG. 4 illustrates a typical remote station generally identified bynumeral 16. Each station includes a shelter enclosure 46 for protectionagainst the elements, because many of the remote stations will belocated at high elevations which are subject to severe weatherconditions. A transceiver and control circuitry 48 are located within awall mounted weather proofed enclosure. A folded dipole antenna 50 isattached to a tower 52 and is connected to the transceiver and control48 via a coaxial cable 54. A solar panel 56 is oriented to receive thesun and is connected to provide power to the transceiver and control 48via wires 58.

Each of the remote stations operate to collect data from severalsensors. In the preferred embodiment, the remote stations are connectedvia cable conduits 60 to a precipitator sensor 62, a temperature sensor64, a snow pillow sensor 66, and a river level sensor 68, each of whichtransmits analog data to the remote station storage. The transceiver andcontrol 48 periodically updates the data stored in response to thesensors 62-68 and then transmits the stored data to the master stationwhen polled.

The receiver of the transceiver 48 is a fixed frequency crystalcontrolled phase shift key (PSK) receiver tuned to the master stationtransmit frequency of 40.53 MHz. Transceiver also includes a phasedemodulator which is a wideband phase-locked loop for detecting ±30degree PSK master station polling. A base band demodulator circuit isused to provide bit synchronization and bit detection of the dataobtained from the phase locked loop. The receiver also employs anabsolute power detector which is used to detect a certain level of RFpower above which the remote station will respond. This insures that theremote transceiver will only be activated by meteors that are strongenough to assure reliable transmission of the remote data back to themaster station.

The transceiver includes a remote exciter comprising a temperaturecompensated crystal oscillator followed by a ±90 degree phase modulator,in order to insure that the remote station has a frequency stability ofbetter than ±5 ppm over a wide temperature range. The output of theexciter is amplified to a 300 watt level by a final power amplifier. Theamplifier is solid state and is operated Class C to obtain maximumefficiency. A transmit/receive switch is mounted on the power amplifierof the circuit and is utilized to allow the use of a single antenna 50for both receiving and transmitting. The switch is normally in thereceive mode. When a master station poll is detected at the remotestation, the transmitter is switched to the antenna 50 and the remotedata is transmitted back to the master station. At the completion of thetransmission, the switch reverts back to the receive location.

The control logic of the circuit 48 operates to provide a number ofdifferent functions. For example, the control 48 receives and analyzesall data from the attached sensors. The control 48 establishes wordssynchronization with the master station by locating the sync field inthe polling signal from the master station. The control 48 determineswhether data is to be sent from the remote station to the masterstation, or whether data is being sent from the master station to theremote station. The control 48 determines whether the remote station isbeing addressed by the polling signal by comparing the address field inthe polling signal against the unique digital address code stored at theremote station. The control 48 controls the transmitter, thetransmit/receive switch, and the transmission of theta. The control 48generates, for use in transmission of data, the preamble including aone's field, a sync character and a station address, along with a textdata stream for transmission and a cyclic redundancy check character fortransmission. The control 48 also maintains four timers including a dutycycle timer which is a one second timer for preventing the remotestation from transmitting more than twice in any one second period. Inaddition, the control 48 controls an acquisition timer used to requirepolling signal reception, both on an initial poll and onacknowledgments, within a 50 millisecond period. Further, control 48controls an inhibit timer which times the period from which a remotestation is inhibited from replying to normal polling signals after thereceipt of an acknowledge command. The control 48 controls an updatetimer which determines when a data update is requested. The control 48further holds and stores for transmission up to 255 bits of sensor datawhich is supplied by an analog-to-digital converter unit in the dataacquisition logic also maintained within the transceiver and controlhousing 48.

The analog-to-digital coverter comprises a dual slope, 12-bitanalog-to-digital converter which performs data acquisition andconversion functions. The analog-to-digital converter automaticallysenses the type of analog data generated by the sensors at the remotestation. The converter is normally in an off state and is powered uponly during a data interrogation cycle to conserve power and increaseoperating life. The sensor data generated by the sensors 62-68 isapplied through the analog-to-digital converter and is inserted into abuffer storage to wait a transmission request from a master station.Additional data bits are updated continuously by a countdown clockcircuit, in order to tell the master station the elapsed time since thelast sensor update at the time the master received the data.

Remote stations have power requirements of 15 ma at +12 VDC. The storagepower source at the remote station 16 includes two batteries which arefloat charged by the solar panel 56 with an output of either 0.3 or 0.6amps with temperature compensation. The remote station transmitter isoperated at plus 28 VDC and includes three 4.5 twelve volt batterieswhich are charged from the twelve volt source at regular intervals. Thesolar panel 56 is constructed on a reinforced fiberglass base and thepanels comprise a Solar Power Model E-12-01365-0.6 which is manufacturedand sold by Solar Power, Inc.

FIG. 5 is a block diagram of the circuitry of a master station accordingto the invention. Four antennas 44a-d, previously shown in FIG. 3, areconnected to a common deplexer 70. The duplexer is connected to fourseparate receiver channels 72, 74, 76 and 78. Each of the receiverchannels is essentially identical and comprises a fixed frequency PSKreceiver tuned to the remote station frequency of 41.53 MHz.

Inasmuch as each of the channels 72-78 are generally identical, onlychannel 72 will be described in detail. Channel 72 includes RF amplifier80, the output of which is applied to a converter or mixer 82 whichprovides mixing from a first local oscillator 84. The mix signal isapplied to an IF amplifier 86 which includes a 10.7 MHz narrow band (15Hz) crystal filter. The output of the IF amplifier 86 is applied to asecond converter or mixer 88 which receives the output of a second localoscillator 90 to obtain a final IF output frequency of MHz. The firstlocal oscillator 84 is used in common between adjacent radio receiverchannels 72 and 74.

Each of the radio channels include an amplitude detector 92 connected tothe output of the IF amplifier 86 to indicate receiver carrier and noisepower. The output of each of the four detectors 92 are connected to ameter driver 94 which drives a signal level meter 96 which is mounted onthe front panel of the system. Meter 96 is useful in determining antennanoise levels.

The output of the receiver 72 drives a demodulator circuit which is usedto demodulate the ±9.0 degree PSK carrier signal. A narrow band phaselock loop 98 receives the output of the receiver and drives a coherentreference signal from the ±9.0 degree signal. The reference signal isthen mixed with the input signal to obtain the baseband signal. At thispoint of the circuit, the baseband signal comprises the desired digitaldata, but includes a substantial amount of noise. In order to provideoptimum protection performance, a post detect matched filter isgenerally utilized. A baseband demodulator 100 receives the digitalsignal and provides bit detection and bit synchronization. The bitdetector is preceded by the integrate and dump matched filter describedabove. The output of the base landing modulator comprises a clock and adata output which are applied to a buffer driver within the controllogic 102.

Control logic 102 receives clock and data output from each of the fourreceiver channels briefly described. The control logic 102 furtherincludes computer interface circuits which provide interfacing to adigital minicomputer 104. In the preferred embodiment, a Nova 3minicomputer manufactured and sold by Data General Corporation isutilized. The computer 104 includes a microprocessor and 16K words ofcold storage, in addition to a real time clock, automatic program loadand power monitor/auto restart. The computer interfaces to a localteleprinter 106 and a modem such as a RS-232C modem 108 to provideaccess to the service center 10. The computer 104 is also connectedthrough a modem 110 in order to provide communication with the othermaster station or stations 112, as previously noted in FIG. 1.

The control logic 102 and digital computer 104 provide transmit key andtransmit data signals to an exciter 114 which comprises a 40.53 MHzcrystal oscillator followed by a ±30 degree linear phase modulator. Themodulator is utilized to minimize the transmitter side band spectrum andapplies the transmission data through a power amplifier 116 to theduplexer 70 for transmission via the antennas 44a-b.

During normal operation, the master station computer 104 automaticallycontrols the operation of the master station. This operation includesturning the transmitter on and off at appropriate times, scheduling andgenerating polling signals, receiving storing and logging all remotedata, and communicating with both the other master station computers andthe service center computer 10. The control logic 102 of the system iscapable of generating any serial data stream commanded by the computer104, with the data rate being determined by the crystal controlled clockgenerated at the master station receiver. Data received by each of thereceiver channels is presented to the control logic 102 which enters itinto the digital computer 104 bit by bit.

Fully automatic operation of the system is provided by a real-time clockwhich provides indications of the time of day and elapsed time. The timeof each master station may be updated at any time by a teleprinter inputor by the service center computer. A power monitor/auto-restart circuitis provided to allow the master station to recover from a power failure.Following a power failure, the computer 104 will automatically reportthe time of the failure to the service center computer 10 and request anupdate of the time of day. Since core storage is provided, the softwareand data is retained indefinitely without required battery backup.

FIG. 6 is a more detailed block diagram of the circuitry of a remotestation generally identified by the numeral 16. As previously noted,each of the remote stations includes a directional antenna 50 which isdirected to a T/R switch 120. The receiver portion of the transceiverincludes an RF amplifier 122 tuned to the master station transmitfrequency of 40.530 MHz. The output of the amplifier 122 is mixed in amixer 124 with the output of a first local oscillator 126. The convertedsignal is applied to an IF amplifier 128, the output of which is mixedby a mixer 130 with frequency from a second local oscillator 132. Theresulting signal is applied to a phase demodulator 134 which comprises awide band phase-locked loop used to detect ±30 degrees PSK masterstation polling signals.

The output of demodulator 134 is applied to a baseband demodulator 136which is utilized to provide bit synchronization and bit detection ofthe data obtained from the phase locked loop. The output of thedemodulator comprises a receiver data and clock signals which areapplied to the control logic 138 for processing. An absolute powerdetector 140 detects the output of the IF amplifier 128 in order todetect a certain level of RF power above which the remote unit willrespond. The detected signal by the detector 140 is applied to thecontrol logic 138 to insure that the remote transmitter is activatedonly by meteors that are strong enough to assure reliable transmissionof the data back to the master station.

Transmit key signals are applied from the control logic 138 to the T/Rswitch 120 to control the operation of the transceiver. Data from thecontrol logic 138 is applied to an exciter 144 which comprises atemperature compensated crystal oscillator followed by a ±90 degreesphase modulator to provide the desired frequency stability of thesystem. The output of the exciter 144 is amplified by power amplifier146 which comprises a 300 watt level amplifier operated class C formaximum efficiency. The output of the power amplifier is connectedthrough the T/R switch 120 to the antenna 50.

The control logic comprises a plurality of integrated circuits of theCMOS logic family. The control logic receives the output of ananalog-to-digital converter in data acquisition logic 150 which isconnected through interfaces 152, 154, 156 and 158 to receive theoutputs of the sensors 62, 64, 66 and 68 described with respect to FIG.4. The analog-to-digital converter comprises a dual slope, twelve bitconverter to perform the data acquisition and conversion function byapplying the sensed data in digital form to the control logic fortransmission to the master station. The converter is normally in the offstate, but is powered up during a data interogation cycle to conservepower and increase operating life. When the control logic 138 commandsan update of data stored in its buffers, the sensor data is insertedfrom the A/D converter into the buffer storage in the control logic 138in order to await a transmission request.

As previously noted, power is provided to the remote stations from atwelve bolt storage battery 160 and a 36 volt storage battery 162 whichis connected to a power control unit 164. In addition, electrical energyfrom the solar panel 56 shown in FIG. 4 is also applied to the powercontrol unit 164. The power control unit thus provides a range of plusand/or minus operating voltages for the power mode station.

As previously noted in FIG. 1, the present invention contemplates theuse of one or more portable field test units 18. A block diagram of atypical portable field test unit 18 is shown in FIG. 7. The heart of theunit 18 comprises a digital microprocessor 170 which enables diagnostictests to be run on the remote station circuitry. The microprocessor thuscontains connectors which may be interconnected with the remote stationcircuitry to provide diagnostic testing thereof. A hand held terminal172 may comprises, for example, a Termiflex HT/2 hand held alphanumericterminal which enables the operator to compose and display alphanumericmessages for transmission to the master and to display alphanumericmessages received from the master station. The display of the terminal172 may display twenty characters simultaneously, while up to 1,000characters may be stored in the circuitry for displaying at operatorsrequests.

The portable field test unit 18 may be connected directly to the remotestation test connector and a message can therefore be loaded into theremote station buffers for transmission to the master station anddisplay of the message on the hand held terminal 172 is provided. Thedata from the various sensors 62-68 may be interrogated in this mannerand displayed on the hand held terminal 172.

The circuitry of the portable field test unit 18 further includes anantenna 174 which is connected to a T/R switch 176. The switch isconnected to a receiver 178 which applies received statements through ADmodulator 180 which stores the data within the microprocessor 170.Remote logic 182 is connected between the microprocessor 170 and thereceiver 178 and applies the transmission signals through an exciter 184and a power amplifier 186 and through the switch 176 to be transmittedvia the antenna 174. An optional test set antenna 188 may be constructedto the remote exciter in order to interrogate and receive at close rangeany closely adjacent remote station. The data transmitted by the remoteexciter 184 is processed by the remote logic 182 and may also bedisplayed on the hand held terminal 172 for verification.

MASTER STATION CONTROL LOGIC

Turning now to FIGS. 8-10, specific circuitry for implementing masterstation control logic 102 to provide interfacing between minicomputer104 and the transceiver portion of the master station hardware ispresented in schematic diagram form. In general, the master stationcontrol logic accepts commands and data from the minicomputer forcontrolling the transmitter portion of the master station transceiverand for generating the transmit data stream, referred to as the probe.In addition, the master station control logic accepts data from themaster station receivers and transfers the information to the computerfor analysis and processing.

Timing for the master station control logic is derived from both thetransceiver and the computer. A crystal derived 16-kHz clock is providedby the receiver portion of the transceiver, as shown in FIG. 5, and isapplied to a divide-by-eight counter 202 through an inverter 204 asshown in FIG. 8. The output of the counter available on output line 206is a 2-kHz clock signal designated TXCLK.

Referring briefly to FIG. 9, timing signals utilized in the masterstation control logic are further derived from the computer and receivedover six input lines designated DS0 through DS5. The binary code presenton these input lines is applied through inverters 207 to a decodercircuit 208 which decodes the binary-coded input address into one offive mutually exclusive outputs. More particularly, the output signalsavailable from decoder 208 are designated DA40, DA41, DA42, DA43, andDA44. The output signals obtained from decoder 208 are inverted by agroup of inverters 210 to provide the complement of each signal. Decoder208 receives an address code from the computer and provides a singleenabling output corresponding to the selection of control logic foreither the transmitter or one of the four receivers.

The master station control logic under computer command controls thetransmitter power by gating transmitter exciter 114 On and Off. Thetransmitter exciter is turned On when the computer executes aninstruction and provides an address on the address but that is decodedby decoder 208 as requesting the signal DA40. This signal is gated bythe computer I/O start pulse, START, at NAND gate 212 in FIG. 8 to setflip-flop 214 and initiate the signal TXKEY through gate 213 which goesto exciter 114. In a similar manner, the transmitter is turned off by acomputer instruction with results in a resetting of flip-flop 214.

With reference again to FIG. 8, the portion of the master stationcontrol logic which relates to the generation of a data stream (i.e.,probe generation) under computer control for transmission at the datarate of TXCLK is presented.

The computer controls probe generation by outputting data words in a15-bit format comprising a 4-bit count field, a parity bit, and a 10-bitaddress. The first four binary bits are the count field, and the lasttwelve binary bits are data. The computer outputs data words with a dataout instruction, designated DOA. The probe data is supplied to themaster station control logic over the lines designated DATO throughDAT15. The four bits of the count field are strobed into a 4-bit bufferregister 216 after inversion by inverters 218. The next twelve bits ofprobe data are strobed into buffer registers 220 and 224 after inversionby inverter groups 222 and 226, respectively. The data word output fromthe computer is strobed into buffer registers 216, 220 and 224 by asignal from AND gate 228. The strobe signal is initiated by theexistence of the DOA instruction from the computer along with a codedinstruction from the computer that is decoded by decoder 208 as aselection of the transmitter. Decoder 208 provides the signal which isapplied as an input to AND gate 228 along with DOA. AND gate 229provides an indication on the SELD bus to the computer that thetransmitter has been selected, but a data out instruction has not beensent.

The four count bits are transferred to means, such as down counter 230,for holding a count of the total number of data bits in the probe andproviding an indication that the last data bit has been provided to thetransmitter. The twelve data bits are transferred into means for holdingprobe data bits from the computer and providing a serial bit stream tothe transmitter. In the embodiment shown, such means comprises 8-bitparallel in/serial out shift registers 232, 234. Transfer of the probedata stream to be transmitted is under the direction of a load signaldesignated NEXT applied to counter 230 and shift registers 232, 234 fromNAND gate 236. The signal NEXT occurs upon the completion of apreviously transmitted data word, as will be hereinafter more completelydescribed.

The signal TXCLK from counter 202 is utilized to shift each bit out ofthe shift registers. TXCLK counts down counter 230 toward zero as eachbit is shifted out of shift registers 232, 234. When counter 230 reachescount zero, indicating that the last bit has been shifted out, a signalis initiated from counter 230 over line 238, which signal is applied asan input to NAND gate 236 to remove the enabling signal NEXT.

If another data word has been loaded into buffer registers 216, 220 and224, flip-flop 240 will be set by a clock signal from AND gate 228initiated by a DOA instruction from the computer. Also, flip-flop 242will be set in response to the setting of flip-flop 240 and NAND gate236 will generate NEXT, permitting the next word to be transferred intocounter 230 and shift registers 232, 234. Flip-flop 240 is and reset bythe occurrence of the signal NEXT. Accordingly, the Q output offlip-flop 240 is in a logic "1" or "true" condition whenever there is adata word in the buffer registers. Flip-flop 242 synchronizes loading ofthe shift registers with TXCLK in order that a new computer word cannotbe loaded into the shift registers in the middle of a bit shifting timeperiod.

The Manchester TXDATA signal for the exciter in the transceiver isgenerated by the combining of the output of shift register 234 and TXCLKin Exclusive-OR gate 244. The combining of the serial data from shiftregisters 232, 234 with TXCLK in Exclusive-OR gate 244 results in NRZcoded data for transmission. The signal available from exclusive-OR gate244 is inverted by gate 246.

The computer generates a probe word in response to a transmit interruptrequest generated by the control logic circuitry. The request enablesignal, RQENB, continuously clocks flip-flop 248 via inverter 250. Whenthe input buffer registers 216, 220 and 224 are empty, the Q output offlip-flop 240 goes to a logic "1" in response to resetting by the NEXTsignal, releasing the reset input of flip-flop 248 and allowing RQENB toset the Q output of flip-flop 248 to a logic "1". The signal availablefrom the Q output of flip-flop 248 is applied to one input of NAND gate252. The other input of NAND gate 252 receives a signal from the Qoutput of flip-flop 254. The signal from flip-flop 254 is an enablinginput to NAND gate 252 which occurs upon the existence of a logic "1" onthe DATA 6 lines simultaneous with a positive-going transition of thesignal MSKO from the computer. If the transmit interrupt is enabled, thesetting of flip-flop 248 will result in the initiation of a TXINT signalreflected by the output of NAND gate 252 going to a logic "0" condition.

The transmit interrupt signal TXINT is applied to priority encoder 256in FIG. 10 resulting in the generation of an interrupt request signalINTR from encoder 256. The interrupt request signal INTR is availablefrom gate 258 following inversion by inverter 260. The signal TXINT hasthe lowest priority of the signals applied to priority encoder 256.Accordingly, a transmit interrupt request will not be generated untilall other requests have been serviced. That is, in addition to theexistence of a logic "0" condition for the TXINT signal, a secondcondition for the generation of the interrupt request INTR is that theinterrupt chain through the priority encoder must not be overridden byother input/output devices with higher priority. Such second conditionis indicated by the signal INTPIN applied to priority encoder 256 whichmust be a logic "0".

Upon the generation of the transmit interrupt request INTR, the computerresponds with a signal INTACK. This signal is applied to one input ofAND gate 262 in the schematics shown in FIG. 10. The second input to ANDgate 262 is the signal INTR. The output of AND gate 262 enables NANDgates 264, 266, 268 and 270 to present at their outputs an interruptcode established in accordance with the binary coded bits available frompriority encoder 256 via inverters 272, 274 and 276. The interrupt coderepresented by the four binary bits designated DATA 10, 13, 14 and 15identifies the interrupt source to the computer as being the transmitinterrupt, whereupon the computer responds by outputting the next probedata word, setting flip-flop 240, and resetting flip-flop 248 whichremoves the interrupt request.

As the master station receiver contains four receivers, each of whichconstitutes an independent data channel, the data reception portion ofthe master station control logic must accept data from a number ofchannels and route it to the computer. Since the received data arrivesin random fashion, any or all of the channels may be active at anyparticular time. As soon as a received signal is detected, the receivergenerates a signal presence indication designated SP. The receivercontinues to receive data until the first "0" in the data bit stream isdetected. Following the detection of the first "0" in the data bitstream, the receiver generates a signal designated SP2 and turns on a2-kHz clock signal designated RXCLK. The signal SP2 from each receiveris supplied to the master station control logic. Since there are fourreceiver channels, the receiver logic is repeated four times. Signals inthe control logic schematic of FIG. 9 are subscripted with anidentifying number corresponding to a particular receiver number.

With reference now to FIG. 9, receiver logic circuits which receive thesignals SP2 from the receivers and generate signal presence hold,designated SPH, are shown. The SPH signals generated by the logic areused to force the recever to continue to present data and clock signalsto the computer, regardless of the existence of a signal presenceindication within the receiver. More particularly, identical logiccircuits 280, 290, 300 and 310 are shown. Since the circuits areidentical in configuration and operation, only logic circuit 310operable in connection with receiver channel number 4 will be describedin detail.

The signal SP24, denoting the signal SP2 for receiver channel 4, startsthe sequence of operation for logic circuit 310 by setting flip-flop312. The Q output of flip-flop 312 is applied to a NAND gate 314connected as an inverter to provide a signal SPH4 to hold the receiverin the receive mode.

Referring briefly to FIG. 9, additional circuitry in the receive logicfor receiving and routing data from the receivers is shown. Again, thecircuitry shown consists of four identical logic circuits 316, 318, 320and 322. Each of these logic circuits receives the 2-kHz RXCLK signalgenerated by the receiver whose data the logic circuit routes.

With reference to logic circuit 322 for receiver channel number 4, witheach low-to-high transition of RXCLK4, a new bit of data is clocked intothe first-in, first-out (FIFO) register 324 as the bit is received. Thedata bits are rippled through the FIFO to the output thereof and areapplied to a data bus 326 through NAND gate 328. Data bus 326 is commonto all of the logic circuits 316, 318, 320 and 322. Data bus 326 makesavailable the received data to the computer for processing. Once data isavailable on bus 326, FIFO 324 initiates a signal releasing the resetinput of flip-flop 330, thereby allowing the first interrupt requestenable signal RQENB supplied from the computer to the clock input offlip-flop 330 to initiate a receiver interrupt signal RXINT on the Qoutput.

A separate interrupt signal is generated by each of the logic circuits.The receiver interrupt signal from each of the logic circuits 316, 318,320 and 322 are applied to priority encoder 256. Generation of theinterrupt signal INTR and the interrupt code available over the linesdesignated DATA 10, 13, 14 and 15 is identical to the generation of thetransmit interrupt previously discussed.

Upon recognizing an interrupt code, the computer responds by outputtingan instruction resulting in the generating of a signal DA44, DA43, DA42,or DA41, depending upon which receive channel requested the interruptfrom decoder 208 in FIG. 9. Therefore, as receiver 4 is undercondsideration in this discussion and data is presumed to be containedon receiver channel 4, the computer checks the output of NAND gate 332in logic circuit 310 shown in FIG. 9 by applying a signal DA44 to oneinput thereof, with the Q output of flip-flop 312 supplying the signalto the other input. In this manner, the computer determines whether thesignal SP24 has set flip-flop 312. If the output signal from NAND gate332 (i.e., SELB) is a logic "0", the computer accepts the data bit onSELD bus 326 as supplied by NAND gate 328 in FIG. 10. The signal DA44 isalso applied to NAND gate 328 and ensures that the channel 4 data is onthe data bus 326.

After ascertaining that the channel 4 data is on the SELD bus 326, thecomputer generates a start pulse which is applied as an input to ANDgate 334 along with the signal DA44. The signal generated at the outputof AND gate 334 clocks the next data bit to the output of FIFO 324. TheDOR output of FIFO 324 is caused to go to a logic "0" momentarily andresets the interrupt flip-flop 330. The process repeats itself until alldata bits have been input to the computer.

After the computer has determined that the proper number of data bitshave been received, the computer generates a DA41 signal and a DOAsignal which are applied to AND gate 336. The output of AND gate 336 isapplied to quad flip-flop 338 to store a code generated by the computerand placed on the computer output bus lines DATA 12, 13, 14 and 15. Thecode generates a signal INHR which is used to reset a particular FIFO.In the case of the discussion of that portion of the circuitry relatingto the channel 4 FIFO 324, the code will generate a signal INHR4. Thissignal is applied to negative-two-input NOR gate 340 in FIG. 9 torelease the signal presence hold signal SPH4 by resetting flip-flop 342,which in turn resets flip-flop 312. Flip-flop 342 is initially reset bya clock pulse from NAND gate 343 in accordance with the application ofDA44 and CLEAR as inputs thereto.

While INHR is high, the receive logic for a corresponding particularreceiver channel is inhibited from receiving. All receivers can beinhibited simultaneously by a clock pulse MSKO applied to flip-flop 344after the D-input to the flip-flop has been set to a logic "1". Theexistence of such conditions causes the Q output of flip-flop 344 toreset all of the flip-flop in device 338 and present a logic "0" on eachQ output.

As will be noted, address decoding for selecting the desired receiverchannel or transmitter is performed by decoder 208 shown in FIG. 9. Thefour receivers and the transmitter have been assigned priorities whereinreceiver 4 has the highest priority, receiver 3 has second priority,receiver 2 has third priority, receiver 1 has fourth priority, and thetransmitter has the lowest priority.

A probe SYNC pulse is generated under software control. NAND gate 346decodes on all ONES probe word output from the computer. The NEXT pulsefrom inverter 347 synchronizes the decode with the probe generation, andlatch 348 extends the pulse to 250 microseconds. Latch 348 comprisesnegative input OR gates 349a and 349b, and receives TXCLK as an input.The PSYNC signal is supplied back to the computer.

REMOTE STATION CONTROL LOGIC

With reference now to FIGS. 11-15 the remote station control logic forgenerating all timing and control functions necessary to receive andanalyze master station probes, including address recognition, and totransmit a reply is shown in schematic diagram form.

Remote station control logic timing is generated by the logic shown inFIG. 11. A two stage oscillator 350 provides a source of internal timingsignals for the control logic. Oscillator 350 includes an inverter 352having a feedback network between the input and output comprising a 256kHz crystal 354. A resistor 356 is in series with crystal 354, and thiscombination is in parallel with resistor 358. In addition, the feedbacknetwork comprises shunt capacitors 360 and 362, each capacitor beingconnected between ground and one side of crystal 354.

The first oscillator stage runs continuously and supplies a clock signalover line 354 to a second stage comprising NAND gate 366. The secondstage functions to gate "on" the 256 kHz clock with the CLKON signalgenerated at the Q output of flip-flop 368. When CLKON is a logic "1",the 256 kHz signal is applied to the clock of 12-bit binary counter 370.

Flip-flop 368 is set to provide a CLKON signal by a logic "1" from ORgate 369 whenever required by the conditions present at its inputs.Signals input to OR gate 369 include ADV from the state control portionof the logic and a signal from NOR gate 371, which itself receives theinput signals SP, TXTEST, CLK and TMR50. Flip-flop 368 is clocked by the62.5 Hz signal from counter 370. The D input is established by theoutput of NOR gate 373 which provides a logic "0" if the control logicis in either a delay state or an idle state. Accordingly, CLKON and the256 kHz clock stay on following setting of flip-flop 368 in thedescribed manner until the control logic returns to State 0 or State 1and counter 370 completes a cycle.

Clock signals ranging from the 256 kHz frequency to 62.5 Hz are madeavailable from counter 370. A 16 kHz clock available from counter 370over output line 372 is applied to the clock input of a 4-bit shiftregister 374. Single bit serial data is input to shift register 374 fromthe Q output of flip-flop 376. A signal RXCLK from the remote stationreceiver is applied to the clock input of flip-flop 376 and the D inputis tied to a logic "1". A negative input AND gate 378 supplies a signalto the Set input of flip-flop 376, which gate receives as inputs a 2 kHzclock signal from counter 370 and a signal presence level SP from thereceiver. The Reset input of flip-flop 376 receives a signal from the Qoutput of shift register 374.

Shift register 374 and flip-flop 376 in combination provide for thegeneration of four, phased timing pulses synchronized with each bitreceived during transmission. These four timing pulses are available asT0, T1, T2 and T3. These timing pulses are used to sequence processingof received data.

A 5-stage Johnson decade counter 380 with a built-in code converterprovides a count of the number of bits processed in each data fieldduring reception or character check transmission. Counter 380 receivesas a clock input signal T0 from shift register 374. A carryout signalfrom counter 380 is applied to the clock input of flip-flop 382. Tendecoded outputs are available from counter 380. These outputs arenormally low and go high only at an appropriate decimal time period. ANDgates 386 and 388 receive as logic inputs the signal available on the Qoutput of flip-flop 382. In addition, AND gate 386 receives the countnumber "2" output from counter 380. AND gate 388 receives the decodedcount number "5" output from counter 380. These two gates provide theTCNT1 signal and the TCNT5 output respectively. The TCNT16 signal isderived from AND gate 390 which receives as input a signal from the Qoutput of flip-flop 382 and the decoded count number "6" output fromcounter 380. A signal ADV6 is derived from AND gate 392 which receivesas logic inputs the signal from the Q output of flip-flop 382 and thedecoded count number "7" from counter 380.

Counter 380 and flip-flop 382 are simultaneously reset by a signal fromOR gate 384. Resetting occurs upon the existence of the designated inputconditions on NAND gate 385, or upon the existence of the conditionsindicated for the combination logic comprising NAND gate 387 and OR gate389. Operation of counter 380 can also be inhibited by a signal to theINH input from OR gate 391. An inhibit signal is issued upon theoccurrence of ALPHAMODE, or upon the existence of a state 1 conditionfor the control logic and a count of eight in counter 380 as detected byAND gate 393.

The timing and field bit count logic is operative in accordance with aparticular control state of the remote station control logic. The remotestation control logic has eight control states ST0 through ST7. Thelogic circuitry for defining the current state of the remote stationcenters around synchronous 4-bit counter 394 shown in FIG. 11 whichalways contains a count from 0 to 7 corresponding to the current controlstate. Counter 394 is clocked by timing pulse T3 from shift register 374and provides a BCD output indicative of the current state of the controllogic. The three most significant bits of the BCD output from counter394 are applied to a BCD-to-decimal decoder 396 and to a data selector398. Decoder 396 codes the count in counter 394 and providesone-of-eight discrete logic levels indicative of the current state ofthe control logic. Data selector 398 selects one-of-seven signals ADV0through ADV6 for advancing the control logic from one state to the next.

The advance signal ADV is supplied as a count enable input P to counter394. Count enable input T for counter 394 is supplied by AND/OR logic395. Both count-enable inputs T and D must be logic "1's" for counter394 to count. Counter 394 is presettable to the logic levels setup atthe counter inputs A, B, C and D. As presetting is synchronous, AND/ORlogic 397 sets up a logic "0" level at the load input LD, and the nextclock pulse sets the Q outputs to agree with the setup inputs.

State 0 of the control logic, designated ST0, is a duty cycle time delaystate during which received signals are ignored. State 0 is used tocontrol the transmitter duty cycle and prevent continuous reception ofsignals during high noise or interference conditions.

With reference to FIG. 12, a circuit for controlling the delay isone-shot multivibrator 400 having external timing components capacitor402 and resistor 404. Multivibrator 400 is triggered by a signal from ORgate 406 which receives as an input a signal present SP and atransmitter test signal TXTEST. A signal ADV0 is available from the Qoutput of multivibrator 400, which is passed through data selector 398to counter 394 to enable counter 394 to advance from State 0 to State 1.The delay provided by multivibrator 400 is for approximately one secondfollowing transmission or an unsuccessful attempted signal acquisition.

State 1 indicated by the signal ST1 is the normal idle state for theremote station control logic. To advance from State 1, a signal presentindication SP by going to a logic "1" and nine ONES in the probe must bedetected as will be more fully described hereinafter. The logiccircuitry for generating an ADV1 signal to advance the control logicfrom State 1 is shown in FIG. 12, with the ADV1 signal being availablefrom the Q output of flip-flop 408. Received data designated RXDATA isapplied to the D input of flip-flop 408 with RXCLK being applied to theclock input.

Signals reflecting the presence of eight consecutive ONES or ZEROES areapplied to negative input OR gate 410. An indication that either allzeroes or all ones have been received will provide a logic "1" outputfrom OR gate 410 which is applied to the D input of flip-flop 412.Timing pulse T1 from shift register 374 in FIG. 11 clocks flip-flop 412to store the output of OR gate 410. The Q output of flip-flop 412 is fedback as an input to OR gate 410 to latch-in an all zeroes or all onescondition. The signal from the Q output of flip-flop 412 is also appliedto the RESET input on flip-flop 414, and upon the reception of eightconsecutive ONES or ZEROES, flip-flop 414 is released to be clocked byRXCLK, establishing a logic "0" on the Q output. Flip-flop 414 causes abit skip holding the flip-flop 408 reset for one bit period to accountfor the ninth one or the phase shift for ZEROES. After the Q output offlip-flop 414 has gone to a logic "0", releasing flip-flop 408, the nextZERO received, which should be the first bit of the SYNC character,establishes the ADV1 signal to cause the control logic to advance fromState 1 to State 2.

The control logic identifies the ONES field in the probe and resolvesthe phase ambiguity introduced by the receiver following a signalpresence indication. The detection of the nine ONES in the probe occursduring State 1 while the control logic is sitting in the idle statewaiting for a probe to be received.

Detection of the ONES field proceeds with received data bits beingshifted into 8-bit shift register 416a, 416b by RXCLK. The received databits are held in shift register 416 and the complement of the data bitsstored in shift register 416 is provided by a group of inverters 418.The complement of the data bits held in shift register 416 are appliedto an 8-input NAND gate 420. If all eight bits of the received data areZEROES, a logic "0" will be provided from NAND gate 420.

Combination logic comprising 4-input NAND gates 422 and 424 along withnegative true AND gate 426 provides logic for detecting the presence ofeight consecutive ONES.

If all zeroes were detected, inverter 428 will apply a logic "1" to boththe J and K inputs of flip-flop 430. The negative-going transition ofthe logic signal on the Q output of flip-flop 412 is applied as a clockto flip-flop 430 causing it to toggle and invert the phase and provide aphase correct signal to the receiver.

Flip-flop 412 can be reset by a signal initiated through NOR gate 434and inverting circuit 436 by flip-flop 438. A logic "1" on the Q outputof flip-flop 438 is established by the condition of the signal presenceSP indication from the receiver. The logic "1" on the Q output isremoved following a a count of eight data bits following the existenceof a signal presence SP indication. That is, for eight bit timesfollowing the existence of a signal presence indication from thereceiver, of a signal presence indication from the receiver, the logicfor generating the advance to State 2 signal is inhibited.

Upon a later return to State 1, as indicated by a logic "1" beingestablished on ST1 at the input of NOR gate 434, flip-flop 412 isreleased to repeat its sequence of operation and again find the firstvalid occurrence of all ONES or all ZEROES.

State 2, ST2, is entered immediately following detection of the nineONES and subsequent generation of ADV1. During this state, the controllogic must detect the SYNC character and valid MODE field. Since thefirst SYNC character bit has been received, only seven more bits are tobe received for the SYNC character and a valid MODE field to be present.An 8-input NAND gate 440 is enabled by the CNT signal or by a CNT 7signal received from counter 380 in FIG. 11. The existence of a validMODE field is checked by Exclusive-OR gate 442. If the SYNC characterand a valid MODE field are present, a SYNC indication is provided fromExclusive-OR gate 444 to generate ADV2 from gate 446 and allowadvancement to State 3.

State 3, ST3, is the control logic state during which the probe addressis received and checked. The 5-bit count field is received firstfollowed by the variable number of address bits. If the station is notbeing addressed, the logic returns to State 1. If the station is beingaddressed, the control logic is advanced from State 3 to State 4.

The probe address field is analyzed in State 3 by the logic shown inFIG. 13.

The 5-bit count field, including the odd-parity bit is received in shiftregisters 416a, 416b. Parity of the count field is checked byExclusive-OR gates 448, 450, 452 and 454. If parity checks, a signalCNTP is generated by gate 454 and applied to the D input of flip-flop456. The signal TCNT 5 from AND gate 388 in the field bit count logic ofFIG. 11 clocks flip-flop 456 to set an address enable signal ADR EN.

The four count bits are applied as inputs to binary counter 458. Thesignal address enable ADR EN enables RXCLK to latch the address bitcount into counter 458 and present at the counter output lines theaddress bit count. If the address bit count is ZEROES or ONES, asindicated by a maximum count signal generated by NAND gate 424 in FIG.12 and made available through inverter 460 to flip-flop 462, ADR ENclocks flip-flop 456 and sets ADV3 for default addressing and immediateadvance to State 4. For any other count, counter 458 sequentiallyselects remote station dress bits A₀ through A₉ from hard-wired addresscode lines 465. The sequentially select station address bits, the outputcode of counter 458 is applied to data selectors 464 and 466. The remotestation address bits are also supplied to data selectors 464 and 466,with address bits A₀ through A₇ being applied to selector 464 and theremaining two address bits, A₈ and A₉, being supplied to selector 466.

As counter 458 counts under the direction of RXCLK, the binary code atthe output of the counter changes sequentially, causing data selectors464 and 466 to select one of the address bits at a time for comparisonat Exclusive-OR gate 468 with each received bit on the RXDATA line.Selected address bits from data selectors 464 and 466 are supplied togate 468 through AND gate 470.

The output of Exclusive-OR gate 468 is a logic "0" if a received databit compares with the corresponding address bit. Any received bits thatdo not compare with the selected address bit results in a logic "1"output from gate 468 which is applied to NAND gate 472. The signal ADREN from flip-flop 456, the synchronized pulse signal T1 from shiftregister 374 in the timing chain logic, and the count five signal TCNT5signal are also applied to NAND gate 472. If a received address bit doesnot compare with the selected address bit in the remote station addresscode, the output of NAND gate 472 causes an address reset signal ADRRSTto be issued from OR gate 474. This signal is applied to AND/OR logic397 in FIG. 11 to cause counter 394 to be returned to State 1.

As each bit of received data is checked, counter 458 decrements until itreaches a zero count indicating that the appropriate number of bits havebeen checked. A carry-out signal from counter 458 is applied to AND gate478 which sets flip-flop 462 to indicate address acceptance by issuingan ADV3 signal.

Address redundancy bits are implimented by applying RXDATA to theappropriate address bit lines Ax to cause Exclusive-OR gate 468 tocompare RX DATA to RX DATA in redundancy bit positions. Accordingly,those bits are eliminated from consideration.

State four, ST4, is a state having two modes of operation. In Mode 1,State four is a null state. It is stepped through in one bit period toState five by the inssuance of the advance signal ADV4 from OR gate 480in FIG. 12. An indication of a Mode 1 operation for the remote stationis provided by flip-flop 482, the Q output of which is applied to ORgate 480. The D-input of flip-flop 482 receives the R1 bit from shiftregister 416a, and flip-flop 482 is clocked by the State three, ST3,signal. If a Mode 2 signal has not been received, flip-flop 482 entersan indication of such mode status. In Mode 2, State four of the remotestation is a state in which a command or text is being sent to theaddressed remote station. The detection of a Mode 2 probe byExclusive-OR gate 448 is entered in flip-flop 482 by the setting of thatdevice upon entry into State three.

If the first bit received in State four of a Mode 2 operation is a zero,flip-flop 484, which receives the R0 bit from shift register 416a and isclocked by TCNT1 from the field bit count logic, remains reset. Thiscondition indicates that duplicate copies of an 8-bit command field arebeing sent. The first eight bits are shifted into the receive shiftregisters 416a, 416b, whereupon CNT8 sets flip-flop 486 to enablecharacter logic comprising flip-flop 488 and Exclusive-OR gate 490. Thesecond copy of the 8-bit command field is received and compared bit bybit with that previously received and contained in shift registers 416a,416b. Any errors in the duplicate command field results in flip-flop 488being set, initiating a command error CMDERR signal and a return toState one. The return to State one is accomplished by the application ofthe command error signal to OR gate 474 in FIG. 13 which generates andADR RST signal that causes counter 394 in FIG. 11 to be preset to theone count.

If all eight bits compare, NAND gate 492 generates a command enablesignal CMDEN to enable decoder 494 in FIG. 12 to permit command signalsto be passed to external devices. OR gate 480 then receives a replyindication, causing an advance of the control logic to State five.

Decoder 494 decodes the second, third and fourth bits of the command andmakes a one-of-eight discrete code lines available to the externaldevice.

If the first bit received in State four of a Mode 2 operation is a one,flip-flop 484 is set and initates an ALPHAMODE to indicate that the textbeing received is to be delivered to the external logic device forprocessing. The external logic device will then generate an ALPHAERR orALPHAADV to return the control logic to State one or advance it to Statefive. If the external logic device does not have the capability toaccept data of this type, all ALPHA lines are left open causing ALPHAERRto be signaled by default.

State five, ST5, is entered whenever the probe has been receivedsuccessfully and commits the remote station to the transmit sequence.The transmit sequence is entered whenever a valid Mode 1 or Mode 2 probethat addresses the remote station is received. When the remote stationswitches from the receive mode to the transmit mode, thetransmit-receive switch 120 in FIG. 6 is moved to the transmit position.Power must be applied to the power amplifier 146, and the exciter 144must be turned on and presented with a data stream.

Logic for turning on transmitter power and setting the transmit-receiveswitch is shown in FIG. 14. A TXPWR signal is generated from NAND gate496 in response to opening of transmit power enable switch 498 andsetting of flip-flop 500 by the ST5 signal. The Q output of flip-flop500 is applied to combination logic comprising NAND gate 502 and ANDgate 504. Accordingly, the power PWR signal from flip-flop 500 gates thesignal presence signal SP to change the timing change synchronizationfrom the receive clock available from NAND gate 506 to the 2 kHztransmit clock TXCLK available from counter 370 in FIG. 11. The controllogic then advances immediately to State six.

State six, ST6, is the transmit reply state during which all data to betransmitted by the remote station is output. In all cases, a field ofnineteen ONES is transmitted followed by a SYNC character and thestation address. This sequence is called the Preamble. The Preambleallows the master station to lock onto the remote station signal andidentify the station. The exciter key signal KEY from flip-flop 508enables the transmission of data.

Remote station transmission proceeds only after the master stationacquires the transmitted signal and identifies the signal source. Asbriefly outlined above, the master station locks onto the remote stationsignal and identifies the remote station by the Preamble that isgenerated by the remote station. Logic for generating the Preamble isshown in FIG. 15 and is enabled when State six is entered.

A five-stage Johnson decade counter 510 and a synchronise 4-bit counter512 cause a nineteen bit time delay, during which TXDATA from AND/ORlogic 514 is held to a logic "1". This time delay permits the masterstation to have 9.5 milliseconds in which to acquire the signal.Following the time delay, flip-flop 516 is set to allow the SYNCcharacter and station address to be shifted out of 8-bit static shiftregisters 518 and 520 and inserted into the data stream through logic514. Shift registers 518, 520 are clocked by TXCLK through NAND gate522, with TXCLK being gated by the signal available from flip-flop 516.Loading of the bits to be shifted out of shift registers 518, 520 intothe data stream is controlled by signal conditions present at the inputsof OR gate 524 which supplies the "load" signal.

The setting of flip-flop 516 further sets flip-flop 526 and enablescheck character generation. The CRCEN signal from flip-flop 526 isapplied to the cyclic redundancy check character generator logic shownin the lower portion of FIG. 15. Both the SYNC character and stationaddress are included in the character check.

The setting of flip-flop 516 further sets flip-flop 528 to provide aTEXT ENABLE. Setting of flip-flop 528 enables the Mode 1 data streamtext to be transmitted. In Mode 2, flip-flop 530 is set by the signalMD2 to by-pass text transmission. In Mode 1, the text is provided by theexternal logic device as DATA OUT to flip-flop 532 via combination logic534. The external logic device continues to supply DATA OUT until theCOMPAR input to flip-flop 530 goes to a logic "1" and sets flip-flop 530indicating the end of text.

Every transmission from a remote station is concluded with a 16-bitcyclic redundancy check character CRCC16. This character is generatedduring data transmission. When flip-flop 530 is set, generating CRCSEL,the sixteen bits of cyclic redundancy check character are inserted intothe transmit data stream through logic 534, flip-flop 532 and logic 514.The check character is formulated in a 16-bit shift register comprisingshift registers 536, 538, 540 and flip-flops 542, 544, 546 and 548. Thecyclic redundancy check character generator logic is supplied with thedata stream TXDATA being transmitted, and with the transmit clock TXCLK.The data stream is applied to Exclusive-OR gate 550 which feedsflip-flop 542 of the shift register chain. The transmitted data bit iscompared with the output of flip-flop 548 at gate 550. The output ofgate 550 is also fed to logic comprising AND gate 552 and Exclusive-ORgate 554. The output of 550 is also applied to a second group of similarlogic comprising AND gate 556 and Exclusive-OR gate 558. The output ofgate 558 is applied to shift register 536.

Following selection of the check character for transmission, counter 380in FIG. 11 is enabled by the CRCSEL signal applied to NAND gate 385.During transmission of the check character, counter 380 counts until atotal count of seventeen has been reached, whereupon AND gate 392 issuesthe ADV6 signal advancing the control logic to State seven.

State seven, ST7, is the transmitter Off sequence. At the completion oftransmission, TX KEY is turned Off by flip-flop 508 in FIG. 14 beingreset through OR gate 560. Simultaneously, flip-flop 500 is clocked andthe signal TXPWR is removed, turning off transmitter power.

After a transmission, the control logic is returned to State one by asignal to counter 394 in FIG. 11 from decoder 396 through OR gate 474,OR gate 476 and logic 397. In State one, the control logic awaitsreacquisition of the master station signal. In normal operation, themaster station will acknowledge receipt of a valid transmission byinserting an acknowledge ACK character in the probe in place of the SYNCcharacter. Only a remote station that has transmitted within 50milliseconds and is addressed by the probe is enabled to receive the ACKcharacter. Flip-flop 562 in FIG. 14 is set by the transmit key signalKEY from flip-flop 508 to provide an acknowledge enable signal ACKEN.Flip-flop 562 is clocked by a signal TMR50 derived from one-shotmultivibrator circuit 564 which is triggered by a signal from OR gate566. Multivibrator circuit 564 includes external timing componentscapacitor 568 and resistor 570 and provides a 50 millisecond time-outfollowing the completion of a data transmission as indicated by a logic"1" on the ST7 input line. A 50 millisecond time-out signal, indicatingthat the remote station has transmitted within the past 50 millisecondsis applied to NOR gate 371 to set flip-flop 368 through OR gate 369 inthe timing chain control logic shown in FIG. 11. The setting offlip-flops 368 initiates CLKON to re-establish operation of the timingchain logic. The ACKEN signal from flip-flop 562 is applied as anenabling input to AND gate 572 in FIG. 12. Eight input NAND gate 574decodes the acknowledge character with an acknowledge pending signalbeing provided by flip-flop 576 in response to a signal from gate 572.After an acknowledge pending signal is sent, the acknowledge characteris checked against the probe address by AND gate 578. The ACK pulse isthen provided as an output to the external logic device where it may beutilized to set some condition in that device.

If the master station signal is still present and the remote station isstill addressed, but there is not acknowledge character in the probe, asecond transmission will be allowed. It is assumed in this situationthat the master station did not receive an error-free firsttransmission. The remote station is, however, prevented from providingmore than two transmissions within the time frame determined by one-shotmultivibrator circuit 400 in FIG. 12 to prevent overheating of the poweramplifier and excess battery drain.

The logic for controlling repeat transmission is shown in FIG. 14. Untila TMR1 signal is applied to flip-flops 580 and 582, a repeattransmission can be provided. However, following the timing out ofmultivibrator 400 following a first transmission by the remote stationas indicated by one-shot multivibrator 564, a TIMOUT signal is generatedfrom NAND gate 584. A CLRST signal from inverter 586 in response to aTIMOUT signal puts the control logic back to State 0.

MASTER STATION PROBING SIGNALS

The master station software is under control of a combinationinterrupt/commutator operating system. Realtime software is executedunder control of the interrupt monitor which receives and identifiesinterrupts, stores status for later return to the operating environmentand delivers control to the appropriate real-time data reception,teletype and modem input/output, real-time clock, and power monitor.

Polling or probe signal generation is initiated whenever the masterstation computer receives a command to initiate a NOMINAL or AD-HOCpoll. The near-real-time (NRT) probe schedule routine determines theappropriate probe sequence to interrogate the required remote stations,turns the transmitter on and initiates the poll.

Once the NRT probe routine initiates the polling, it becomes theresponsibility of the real-time (RT) probe generation routine togenerate the continuous, uninterrupted probe--including insertion of theacknowledge sequences when remote replies are received. The RT proberoutine supplies the appropriate data words to the probe generationhardware to cause it to generate the required probe. The probe isrepeated continuously until the NRT probe schedule changes or deletesthe probe requirements.

When the master station receiver receives remote data, it presents eachbit, as it is received, to the master station computer. At this pointthe real-time routine performs the preliminary data processing. First,the software finds the synchronization character to verify that there isa proper locking on the incoming data. The next ten bits are thenreceived and assembled as the unique address of the transmitting remotestation. The software will multiplex the outputs of two receiverssimultaneously. In the use of four receivers, the hardware logic willgate two receivers at a time to the computer, inhibiting the other tworeceivers for the duration of the reception.

Once the master station has identified the transmitting remote station,it references that remote's status table to determine the formate andlength of the data stream being transmitted by that remote. Theappropriate number of data bits are then received and stored for furtherprocessing. And as each bit is received, it is entered into thecalculation of the cyclic redundancy check character. At completion ofdata reception, including reception of the 16-bit check character, thecheck character calculated by the RT receive software will be zero ifall bits, including the remote address, have been received correctly. Ifthe data has been received correctly, the real-time probe routingimmediately sends an acknowledgment to the transmitting remote and thenear-real-time software is directed to continue processing, logging, andstorage of the received data. If the data has not been receivedcorrectly, it is discarded.

A real-time power monitor routine will be immediately called any timethe processor power monitor detects a power drop. This routine will turnoff the transmitter and store current operating status. It will thenprotect core storage and halt the computer to prevent data loss duringthe immanent power failure.

When power is reapplied to the system, the processor power monitor wilautomatically restart the computer in the power monitor routine. Systemstatus will be re-established and, after appropriate time delay fortransmitter warm-up, the transmitter and probe generation will be turnedon. Also, the service center 10 will be notified of the time of thepower failure and it will be requested to update the computer'sreal-time clock so the central can re-establish time-of-day andscheduling requirements.

The master station polling or probing signal comprises four data fieldstransmitted continuously in search for a communications link to a remotestation. As shown in FIG. 16, the first data field consists of nine ONESand is used by the remote stations to determine proper bit phase. Thisis necessary as the transmitted data is Manchester coded and has anambiguity between the first half and second half of each bit. Therefore,each remote station that hears the master station probe must search forthe nine ONES field first. If it finds a ZEROES field instead, bit phaseis known to be reversed.

As shown in FIG. 16, the second data field in the probe contains a sixbit SYNC character and a two bit mode indicator. The remote station mustreceive the SYNC character immediately following the ONES field toverify that it has proper synchronization with the transmitted probe.The mode bits must either be 01 or 10 for Mode 1 or Mode 2,respectively. Mode 1 requests data and Mode 2 is a command feature.

The third data field, as shown in FIG. 16, contains the address of theremote station or stations that are to respond to the probe. The addressfield contains three parts. First is a four bit count field whichspecifies how many address bits are being transmitted. Second is aparity bit which provides odd parity over the count field as a checkthat the count was received correctly. Third are the address bits. Theremay be from 0 to 10 address bits as specified by the count field, andall address bits transmitted must agree with the appropriate bits in theremote station's address in order for that remote to respond to theprobe. For example, if three address bits are transmitted, they must beidentical to the most significant three bits of the remote station's tenbit address for that remote to be addressed.

If no address bits are transmitted, every remote station is, by default,addressed. If ten address bits are transmitted, they must compare withall ten of the remote station's address bits, and thus only one remotecan be addressed. It may thus be seen that the present polling techniqueallows for selective polling of all remote stations, a selected group ofremote stations or an individual remote station.

The fourth field is present only for Mode 2 probes containing commands.If the first bit of the fourth field is a logic zero, then two copies ofa seven bit command are being sent and the remote station must receiveand compare both copies of the command in order to accept it. If thefirst bit of the fourth field is a logic one, then the data contained infield four is to be routed to an external device. The external devicemust process the data and make the accept/reject decision.

Whenever the remote station receives correctly the master stationprobing signal and determines that the remote station is beingaddressed, it will transmit a reply to the master station. The datafield of the remote station reply is shown in FIG. 17. The reply willalways contain the preamble and the 16-bit cyclic redundancy checkcharacter (CRCC-16). In Mode 1, it will also contain the data being heldin the remote station.

As shown in FIG. 17, the remote preamble consists of a nineteen bit ONESfield, used to provide a time delay to allow the master station time toacquire the signal, a SYNC character, used to allow the master stationto verify proper frame synchronization, and the remote's unique 10-bitaddress, identifying the remote to the master station.

In Mode 1, the text is transmitted next. Text may consist of from 1 to255 bits and is data that is entered from the sensors associated withthe remote station. In this case, forty-eight bits are sent for foursensors plus four bits clock count. The response is completed when theCRCC-16 code is transmitted. The CRCC-16 is generated over the addressfield and the data field and thus verifies both remote identification(even in Mode 2) and correct reception of the text.

The master station may acknowledge proper reception of the remotestation data by responding to the reception with the ACKnowledgesequence. The field for this sequence is shown in FIG. 18. This sequencecomprises a field of thirty ones, during which the remote stationreacquires the signal after transmitting, and eight-bit ACK character,and a remote address. Only a remote station that has transmitted in thelast 50 milliseconds may accept the ACK and inhibit transmitting for thepreset time of the inhibit timer (e.g., for 1/2 hour). Once inhibited, aremote staion may only respond to a master station Mode 1 signal if itis uniquely addressed or the probe address field (field three) is allONES, indicating a universal poll wherein all remote stations mustresond.

The remote station addressing scheme used by the present pollingtechnique provides a great deal of flexibility in grouping and selectingremote stations that will respond to any given probe. Each remotestation can be addressed uniquely or as part of various groups. Groupsize can vary from one (unique) to 1024 (all) according to the number ofaddress bits used.

As shown in FIG. 16, the address format used contains from 5 to 15 bitsin the address field. Each remote station has a unique 10-bit address (0to 1023). In order to response to a probe, a remote station must find inthe probe its address or the most significant bit portion of itsaddress. The count portion (CNT) of the probe address fields specifiesthe number of address bits transmitted. A parity bit, P, is transmittedas a check bit on the count. If the count is zero, CNT=0, then there areno address bits and all remotes are addressed. In this case the totaladdress field is five parts, CNT+P.

If CNT=1, there is no address bit and a remote station is addressed onlyif its most significant address bit is equal to the bit transmitted.Since there are two possible states for the transmitted address bit (0or 1), half of all possible remotes will be addressed, defining twogroups: remotes 0000-0777 will respond to 0 and remotes 1000-1777 willrespond to 1. FIG. 19 shows this example and other examples as theaddress is increased in length, for CNT=2, 3, 4, 5 and 6. In each case,the number of bits in the transmitted address increases by 1, doublingthe number of groups addressed. For example, CNT=2 has groups 00, 01,10, and 11 and cuts in half the number of remotes in each group. WhenCNT=2, the two most significant bits of a remote's address must matchthe transmitted address. This structure continues to CNT=10 (addressfield size 15) where there are 1024 groups of remotes with one remote ineach group (e.g., unique addressing).

In the above description, it was assumed that all remote stations mustbe addressed by their exact address in order to respond to a receivedprobe. As shown in FIG. 19, this results in each remote being includedin one and only one group at each level of addressing. However, it maybe desired to have a remote reply to more than one probe at any givenlevel. For example, assume the probe contains CNT=2, defining fourgroups as shown in FIG. 19. Remote 0000 would respond only if group 00were being probed. But assume it is desired to have this remote stationalso reply to a group 01 probe. Then the remote station would have toignore the second address bit in order to find an address to which itcould respond. This can be accomplished by setting the second bit of theremote receive address to a "don't care" condition, i.e., the remotedoes not care whether the second bit of the received address is a one ora zero, creating the addressing redundancy that allows the remote toanswer either group 00 or group 01 probes. Any of the first five bits ofa remote's receive address may be made redundant.

However, the transmit address always remains unique. The receive addressredundancies and the transmit address are determined by the address plugat each remote and must be assigned as part of the system definition andcan only be changed by changing the address plug at the remote.

The primary application of the redundancy bits in the remote receiveaddress shown in FIG. 16 is to allow flexibility in assigning remotestations to master stations in a multiple master station system such asshown in FIG. 1. Normally, a remote station that is "in range" of two ormore master stations will be assigned primarily to one particular masterstation. That master station will probe an address to which that remotestation will reply while the other master station will probe an addressto which that remote station will not reply. Such a remote station is"exclusively" addressed by a particular station. Of course, eithermaster could use a more general address or the other master's probe tocommunicate with remotes not primarily assigned to it. If it is desiredto have a remote assigned to either of the master, the remote's receiveaddress is made sufficiently redundant to allow it to respond to eithermaster. In effect, this remote station now receives double coverage.

Once a remote station is assigned to a master station or stations, it isdesirable to be able to assign the remote station to answer specificprobes that that master station will use. If each master uses four probepatterns to address all of the remotes assigned to it, then it may bedesirable to assign each remote exclusively to answer one probe, toanswer any two of the probes, or to answer all of that master's probes.Selecting redundancy for the probe bits that control these replies willallow each remote to perform as required.

When the present system is installed at a particular field site, thereare several site factors which must be taken into consideration. First,if a station is line-of-sight to any of the master stations, then itwill be probed uniquely in sequence with other line-of-sight remotes.

If a remote station is not line-of-sight and is located in a good area,distant from other remotes, it may be addressed in any manner desired.If this remote station is within a few miles of another remote station,then master assignments and probe assignments of all remotes in closepriximity must be exclusive so that two or more remotes will notcontinue to answer the same probe simultaneously, thus nullifying eachother. Finally, if the remote site has a low performance factor due tolong range or poor horizon, the remote address must be provided withsufficient redundancy to allow it to respond to a large percentage ofthe probes and any of the masters that it may hear.

Another important aspect of the invention is that the portable fieldtest units 18 can also communicate and be probed by either of the masterstations. The portable field test units can transmit a 16 charactermessage in response to any mode 1 probe from a master station thataddresses the portable field test unit or a remote station through whichthe portable field test unit is transmitting. The first seven bits ofthe portable field test unit receive address are redundant, so theportable field test units are virtually always addressed. When aportable field test unit is utilizing a remote station transceiver, aportable field test unit address plug is normally inserted into theremote station transceiver to insure response to all probes.

The portable field test unit message reply format is shown in FIG. 20.As in all remote station replies, the ones field and the SYNC characterare transmitted first, and then the portable field test unit address.Bit 35 is a one to indicate that a ASCII message follows. Bit 36 is adummy bit generated by the portable field test unit microprocessor. Thesixteen 7-bit (6-bit plus parity) ASCII characters follow and the CRCCcharacter verifies the data stream. The standard ACK sequence from themaster station allows the portable field test unit to delete thetransmitted message from its transmit cue.

FIG. 20 also illustrates the probe format for transmitting in ASCIImessage from a master station to a portable field test unit. There aretwo techniques for transmitting this message. If a dedicated probe istransmitted which uniquely addresses a portable field test unit with amode 2 probe, bit 33 is set to indicate the mode 2 information is ASCIItext rather than a command. The sixteen character message follows, andthe hold probe is transmitted continuously until the portable field testunit ACK acknowledge is received to verify reception. An alternatetechnique transmits the message only in response to a receive messagefrom the portable field test unit in question. The single messagereplaces the normal ACK to the portable field test unit and the onesfield is extended to thirty ones to allow the remote station to acquiresignal.

The performance of a meteor burst communication network is primarilydependent upon the time interval between successive meteor burstreflections. The diurnal and seasonal variations of the average numberof bursts are natural phenomena and cannot be altered. However, thenumber of usable bursts, or reflections, can be improved by optimumlyadjusting the various physical system parameters.

The occurence of sporadic meteors are random events and the intervalbetween these occurrences are Poisson distributed. Therefore, theprobability that a meteor burst reflection will occur after a certaintime t_(o) can be expressed by: ##EQU1## where n is the average numberof meteor burst reflections that occur during one hour. Therefore, for ameteor burst network of 160 remote stations, the communication link witheach remote station will be an independent event and the probability ofcommunicating with all 160 stations, during a given time t_(o) can beexpressed as: ##EQU2## The probability of communicating with each remoteis, therefore, both burst (n) and time (t_(o)) dependent. The systemdesign should maximize these values so that the percentage of remotestations that can be expected to respond within the prescribed timeperiod will be at a maximum.

The system response time can be predicted using the cumlulative binomialdistribution. The expression used to calculate the probability forexactly x out of n remote stations successfully responding is: ##EQU3##where n=total number of stations

x=total number of stations responding successfully

p=probability of single station success

q=1-P

The probability of "X" or more stations successfully responding is:##EQU4##

The system performance as a function of message length is defined by theequation below: ##EQU5## where Tx=message length in seconds

Td=burst time constant, seconds

M=system constant

n=usable meteors per hour

COMPUTER PROGRAM FLOW CHARTS

As previously noted, each of the master stations 12 and 14 includeprogrammable digital computers which operate to control various aspectsof the probing and reception of data. Due to the large number of remotestations associated with the master station, the acquisition of data bythe polling of remote stations is an important aspect of the presentinvention. The present polling technique enables the selectiveinterrogation of any selected remote station, or any selected group ofremote stations, and includes techniques for eliminating theinterferences from other acknowledging remote stations and due toweather conditions and the like.

FIG. 21 is a flow diagram of a polling interrupt routine that determineswhat type of a polling sequence is presently being transmitted by aremote station.

The program initiates at 600 and determines at 602 whether or not themaster station is presently generating an acknowledging signal to aremote station. If the system is generating an acknowledge signal, thenext word is obtained at 604 and is output at 606 and the system returnsat 608 to finish last state at 600.

If an acknowledge signal is not presently being generated by the masterstation, a decision is made at 610 as to whether or not a command signalis being generated by the master station.

As previously noted, master stations may transmit an eleven-bit commandsignal to selected remote stations in order to turn selected sensors onand off, to collect data at a different rate or to control the operationof various input/output units attached to the remote station. If thecommand signals are being generated by the master station at 610, thenext word is obtained at 612 and the word is output at 606.

If the master station is not presently generating a command signal, adecision is made at 614 to determine whether the system is in theprocess of sending a message to a portable field test unit. If so, thenext word of the portable field test unit method is obtained at 616 andis transmitted at 606. If no message is being transmitted to theportable field test unit at 614, the system is doing a conventionalprobe and the next character of the probe message being transmitted isretrieved at 618 and is output at 606 for transmission. Normally, theword retrieved at 618 will be a Mode 1 probe in either a nominal poll,an ad-hoc poll or a background poll. As previously noted, a nominal pollcomprises the generation of a probing signal which encompasses all ofthe remote stations associated with a master station. An ad-hoc probecomprises a polling sequence for communicating with a specific selectedgroup of remote stations with as little communication as possible withthe remainder of the remote stations. A background polling sequencecommunicates with specific remotes when nominal or ad hoc polling arenot in progress, and is typically used last to allow communications withportable field testing units.

FIG. 22 illustrates a flow diagram of a computer program routine forautomatic probe control (APC) for controlling the generation of variousones of the three types of polls. As will be shown, this program enablesthe polling sequence being transmitted to be varied by adding orsubtracting bits in response to responses listed as a result of a probe,in order to introduce new probes gradually to allow line-of-site remotestations to respond independently, or to add or delete extra addressbits as noise. Sporatic "E" or auroral reflections are detected by thesystem. The system thus automatically optimizes the polling sequencesbeing transmitted both before the probes are initiated and during actualtransmission of the probes.

Referring to FIG. 22, the APC program is initiated at 630. A decision ismade at 632 as whether or not a nominal poll is active. If a nominalpoll is active, the nominal control table is selected at 634. If anominal poll is not active, a decision is made at 636 as to whether ornot an ad hoc poll is active. If "yes", the ad hoc poll table isselected at 638. If not, a decision is made at 640 as to whether or nota background poll is active. If "yes", a background control table isselected at 642. The control tables for the various nominal, ad hoc andbackground polls are stored in the computer under operator control andmay be varied as desired. If a background poll is not determined to beactive at 640, the program exits at 644.

A decision is made if a nominal, ad hoc or background poll is active at646 to determine what the noise environment is. At 646, a decision ismade as to whether any good messages have been received from remotestations in the last 5 seconds. If so, the probe is determined to begood and the program increments to decision point 648. If at least onegood reception from a remote station has not been received in the last 5seconds, it will be assumed that either the noise envvironment isexcessively high or the conditions are such that numerous remotestations are attempting to communicate at the same time and thereforenone of them are successful in being able to communicate with the masterstation because they are jamming one another. An accumulator is providedin the digital computer at the master station which counts each time agood message is received. Each 5 seconds it is reset. At 646 the countin the accumulator is detected.

If at least one good reception from a remote station had been detectedin the last 5 seconds at 646, a decision is made at 648 to determinewhether or not the minimum number of bits exists in the probe address.If "yes", the program increments to B, to be subsequently described. Ifnot, the probe address is incremented at 650. Normally, if a remoteprobe is being transmitted, only two probe bits are being transmitted inorder that as many remote stations in the field as possible may bepolled. Therefore, if extra bits are determined at 648, the programoperates to delete extra bits. After the probe address has beenincremented at 650, a decision is then made at 652 as to whether or notthe address cycle has been accomplished. If so, one bit is deleted fromthe probe address at 654 and the system cycles to the calculation of newprobe words at 656. If the address cycle is not accomplished at 652, theprogram increments to the calculation of new probe words at 656.

If it is determined at 646 that no good receptions have been received inthe last 5 seconds, a decision is made at 658 as to whether or not themaximum number of bad messages stored in the computer is exceeded.Normally, in the preferred embodiment the maximum number is set at ten.If this maximum has not been exceeded, the decision at 648 is next made.If the maximum number of bad message has been exceeded at 658, adecision is made at 660 as to whether or not the maximum number of bitsare in the probe address. If "yes", the system increments to thedecision at 648. If not, an additional bit is added to the probe addressat 662. The addition of an additional bit further defines the remotestations being probed by the system and tends to improve thesignal-to-noise ratio of the probing technique. The loops of adding orsubtracting bits are continually reiterated so that the probing addressis continually updated and optimized in view of rapidly varyingenvironment characteristics.

At 656 in FIG. 22, the new probe words determined as a result of theprevious addition or subtraction of bits are determined from a tablestored in the memory of the computer. A decision is made at 666 aswhether or not a new probe word has an excessive number of bits. If so,the program increments back to the decision at 648 in order that thebits may be eliminated. If not, the probe words are posted for the clockinterrupt subroutine at 668, and a probe address is obtained from storedtables at 670. The table address is calculated at 672 to which remotestations will reply. This step determines whether or not a remotestation exists which will answer the address calculated at 670. Adecision is made at 674 as to whether or not there is a requirement toobtain data from the remote station whose station address has beencalculated. If so, the progam exits at 676. If not, a decision is madeat 678 as to whether or not a minimum number of bits are in the probe.If so, the total probe will be ineffective since data has been receivedfrom all remote stations identified under that address. Therefore, theprobe is deleted at 680 and the next probe is scheduled at 682 and theprogram exits at 684. If a minimum number of bits are not in the probeat 678, the routine reiterates beginning at the decision point 634.

The schedule (SCHED) sub-routine runs each minute of operation of thesystem. The schedule sub-routine allows the system to probe certainpatterns of remote stations for certain periods of time. For example, ina typical ad hoc poll, the system might probe for one subset of remotestations for eight minutes and then change the probe to address anothersubset of remote stations for eight minutes. The SCHED sub-routine timesout the eight minutes and keeps track of which probes have been runningfor how long and allows the system to change from one probe to anotherin a cyclic manner. The system comes on each minute and checks theprobes that are scheduled and determines which should be running, andwhether or not the one that is currently running is timed out. If theprobe has timed out, then the schedule advances to the next probe andwhen that probe is initiated, the schedule sub-routine calls the postroutine, to be subsequently described in FIG. 24, which places the probein transmission.

The program is initiated at 690 and the highest priority table sectionis selected at 692. The various probes are provided with differentpriorities, with nominal polling having a higher priority than ad hocpolling, and ad hoc polling having a higher priority than backgroundpolling. Therefore, the system tends to sehedule the probe that ishighest on the priority list. A decision is made at 694 as to whether ornot a probe is presently active. If not, the system increments the loop696 to steps to be subsequently described. If a probe is active, theremaining time for the probe to run is decremented at 698. A decision ismade at 700 as to whether or not the time allotted for the probe is up.If so, the active probe flag is set to zero at 702. If time is not up,the routine advances to the decrementing of all time until schedule(TTS) words at 704. A decision is made at 706 as to whether or not theTTS word has been set to zero. If "yes", the pending flag is set at TTSequals the desired period of operation at 708. If the TTS word is notset to zero, a decision is made at 710 as to whether or not the activeprobe flag has been set to zero. If not, the system exists to return at712. If the active probe flag has been set to zero, a decision is madeat 714 as to whether or not a new probe is pending. If not, the programroutine exits at 712. If "yes", the post program is called at 716.

The post program is illustrated at FIG. 24 and serves to determine thehighest priority probe which is next to be run and will post the probefor running.

The post program is illustrated in FIG. 24 and initiates at 720. At 722,the active probe word is picked up from the accumulator AC2 and storedin the active probe storage. The probe pending flag is cleared at 724which was set by the SCHED routine as shown in FIG. 24. In order togenerate the actual address and probe bits to be run, the post programthen adds a parity bit to the address count at 726 and merges the countfor the probe generator at 728.

At 730, the count is posted in the probe word and the count is mergedfor the address at 732. The address is posted in the probe word at 734and the program returns at 736. The post routine thus puts up theaddress bit it desires to be transmitted. The automatic probe control(APC) routine shown in FIG. 22 may thus operate upon the addresssequence to add or subtract bits in a necessary manner to meet withcurrent signal and noise-to-ratio requirements and the like.

The initiate nominal poll (INTNP) routine sets up the system for nominalpolling and scheduling by the schedule program to poll all remotestations associated with a particular master station. The INTNP programis initiated whenever an INT command is received from the centralservice center 10. Thus, at 642 the INT signal is logged on theteleprinter associated with the computer at the master station. At 744the Young routine is called which determines any data stored in corewhich is less than a half-hour old. If such data is available, noadditional communication is necessary with the remote stationsgenerating the data. Thus, nominal polling is accomplished from thispoint on with regard to remote stations which have transmitted data overone-half hour ago.

At 746 the nominal polling is set to be active and an "END" message isenabled at 748. Four thirty minute, two-bit probes are set up at 750 andthe probes are scheduled at 752. The APC routine is called at 754 inorder to subtract or add bits from the address, as produced by the postroutine, and the INTNP routine exits at 756.

FIG. 26 illustrates the ad hoc processor (HOCP) routine which generatesthe polling sequence for ad hoc polling. The routine initiates at 760and causes the Young program 762 to eliminate communication with anyremote station which has transmitted a valid response within the lastthirty minutes. The ad hoc SCHED tables are initialized at 764 in orderto determine the best probes to use to communicate with the remainingremote stations. At 766 a list of remote addresses is generated from thestored ID's in the computer storage. The addresses of the remotestations involved are thus generated at 766.

A decision is made at 768 as to whether or not there are any additionalremote stations to communicate with in the ad hoc program. If not, theprogram exits at 770. If there are remote stations left to becommunicated with, a decision is made at 772 as to whether or not thereare more than four remote stations to be communicated with. If so, theprobes are generalized at 774 to require only four probes in order toenable completion of the ad hoc probing in the necessary half-hourperiod. The length of the address generated at 774 is dependent upon thenumber of remote stations to be communicated with as compared with theamount of time available. For example, if only four remote stations arenecessary to be addressed, each of the remote stations could be probedfor eight minutes a piece. If ten remote stations are to be communicatedwith, each one could only be individually probed for three minutes.Therefore, at 774, the system compresses the available addresses by aeliminating bits until the stations are grouped wherein they may beprobed by only four discrete probes in order to allow the necessary timefor each probe during a thirty minute period.

If four or fewer remote stations are required to be contacted, adecision is made at 776 as to whether or not only one probe is required.If so, a single continuous probe is set up at 778 and is sent to theschedule at 780. If not, a decision is made at 782 as to whether or nottwo probes are required. If so, two commutating sixteen minute probesare set up at 784 and are scheduled at 780. If more than two probes arerequired, a decision is made at 786 as to whether or not three probesare required. If so, three commutating eleven minute probes are set upat 788. If more than three probes are required, a decision is made at790 as whether or not four probes are required. If so, four commutatingeight minute probes are set up at 792. After the probes are scheduled at780, the ad hoc poll is set active at 782 and the program exits to theschedule at 784.

FIG. 27 illustrates a flow diagram of the definition (DEF) routine whichdetermines the addresses of the remote stations in the field to becontacted. The routine initiates at 790 and a decision is made at 792 asto whether or not two parameters have been defined. When the operatordefines the routine, the operator types in commands on the teletypeassociated with the computer. One parameter is the station address,which is the actual address used in the probe to communicate with aparticular remote station, and the second parameter is the ID which isused to identify that remote station to the central service center 10.If two parameters have not been input, the routine exits at 794 and anindication is provided calling for the addition of the necessaryparameters. If the two parameters are present at 792, the remote stationID is obtained at 796. A decision is made at 798 as to whether or notthe ID is within the system size. If not, the program exits at 800 and acommand is presented to correctly identify the ID. If the ID size iscorrect, a decision is made at 802 as to whether or not the ID has beendefined. If so, a decision is made at 804 at to whether or not the ID isthe same as the stored definition. If not, the routine exits at 806 toallow the correct definition to be input. If "yes", the systemincrements at 808 to a termination of the program.

If the ID is not defined, the address is added to the stored table at810. A status block is then added at 812 for the defined remote stationand the program exits at 814.

The DEF program determines the addresses of the remote stations. Aspreviously noted, each remote station in the field has a unique addresswhich is transmitted to the master station when the remote stationresponds to an appropriate probe. The address transmitted from theremote station determines which probes the station will respond to. Thisis the address command that defines to the software which remotestations are stored in the system. The stored responses in the softwareare also equated to an ID which is utilized to identify the remotestation in the central service station 10. In the polling technique,when a station is defined that identifies the address of the remotestation and provides a status for that remote station.

It may thus been seen that the present system provides a meteor scatterburst communications system which enables a large number of remotestations to be remotely located in generally inaccessible areas andinterrogated to automatically transmit data to one or more masterstations. The particular polling technique of the system enables any oneof any selected group of the remote stations to be selectively polled atany time according to any predetermined schedule. The system enables thesimultaneous reception of radio signals reflected from meteor trailsfrom a plurality of remote stations, without undue interference. Withthe present system, data can be readily obtained regardingmeteorological and weather conditions at sites heretofore generallyinaccessible for the repeated collection of such data, thereby enablingmuch more accurate forecasting of environmental conditions. The presentpolling technique enables remote stations to be selectively polled withautomatic compensation for noise and conditions introduced by variousfactors and enables a continuous standard of signal to noise to bemaintained in order to obtain meaningful data in all or in many varyingconditions.

The present polling technique is controlled by a programmable digitalcomputer at each master station. While it would be understood thatvarious programmable digital computers or hardwired digital systems maybe utilized to perform the described functions of the invention, in thepreferred embodiment a Nova Three Minicomputer manufactured and sold byData General Company has been utilized. The computer has been programmedwith Data General computer language and a program listing for performingthe previously described routines relating to remote station polling isattached herewith as Table I. ##SPC1## ##SPC2##

Whereas the present invention has been described with respect tospecific embodiments thereof, it will be understood that various changesand modifications will be suggested to one skilled in the art, and it isintended to encompass such changes and modifications as fall within thescope of the appended claims.

What is claimed is:
 1. A meteor burst communications systemcomprising:at least one master station, a plurality of remote stationsspaced at locations remote from said master station, said master stationincluding a radio transmitter for transmitting probing digital radiosignals having address portions of selectively variable length, saidprobing digital radio signals being directed from said master stationfor reflection from meteor vapor trails to said remote stations, each ofsaid remote stations including a radio receiver for receiving saidreflected probing digital radio signals from said master station, eachof said remote stations including address recognition circuitry having apredetermined digital address sequence stored therein and further havingmeans to compare the received address portions with said stored digitaladdress sequence, at least one sensor of physical characteristicsconnected with each said remote station, and each remote stationincluding a transmitter for transmitting digital data representative ofthe output of said sensor to said master station via reflection from ameteor vapor trail if said received address portion compares with saidstored digital address sequence in accordance with predeterminedcriteria based upon the length of said received address portion.
 2. Themeteor burst communications system of claim 1 and further comprisingmeans in said master station for transmitting an acknowledge signal toeach remote station which answers said probing digital radio signalstransmitted by said master station.
 3. The meteor burst communicationssystem of claim 2 and further comprising:means in each of said remotestations for inhibiting further operation of said remote stations for apredetermined time interval after the reception of said acknowledgesignal.
 4. The meteor burst communications system of claim 3 and furthercomprising:address means at said master station for transmitting anaddress to an inhibited remote station to override said means forinhibiting and allowing communication between said master station andsaid remote station.
 5. The metero burst communications system of claim1 wherein said address portion of said probing digital radio signals hasa predetermined variable length up to n bits, the less the number ofbits contained in said address portion the greater the number of remotestations which will respond to said probing digital radio signals. 6.The meteor burst communications system of claim 5 and furthercomprising:storage means in said master station for maintaining a recordof which remote stations have responded to a probing digital radiosignal having an address portion encompassing a plurality of remotestations, and means responsive to said storage means for increasing thenumber of bits in said address portion to exclude probing of remotestations which have already responded.
 7. The metero burstcommunications system of claim 1 and further comprising:at least twospaced apart master stations each transmitting a different probingdigital radio signal, means for setting said stored digital addresssequence at a remote station such that said remote station responds toeither of said master stations.
 8. The metero burst communicationssystem of claim 7 and further comprising:a service center having meansfor communicating with and controlling operation of each of said masterstations.
 9. The meteor burst communications system of claim 7 andfurther comprising:means for communicating between said master stationsto apprise each master station of the probing progress of the othermaster station.
 10. In a meteor burst communications system wherein amaster station intermittently contacts a plurality of remote stations,the combination comprising:means for transmitting a digital probingradio signal having an address field of n bit capacity, means forcomputing a digital address having a number of bits up to n bitsdependent upon a desired number of remote stations to be contacted, andmeans for applying said digital address to said transmitting means fortransmission in said address field.
 11. The combination of claim 10 andfurther comprising:means responsive to the reception of responses fromthe remote stations for varying the number of bits in said digitaladdress.
 12. A meteor burst communications system comprising:at leastone master station, a plurality of remote stations spaced at locationsremote from said master station, said master station including a radiotransmitter for transmitting probing digital radio signals, said digitalradio signals being directed from said master station for reflectionfrom meteor trails to said remote stations, each of said remote stationsincluding a radio receiver for receiving said reflected probing digitalradio signals from said master station, at least one sensor of physicalcharacteristics connected with each said remote station, each remotestation including a transmitter for transmitting at a specifiedfrequency digital data representative of the output of said sensor tosaid master station via reflection from a meteor trail, said masterstation including a plurality of antennas connected to discrete radioreceiving channels for simultaneously receiving digital data from aplurality of said remote stations, means connected to said radioreceiving channels for storing the received digital data for subsequentuse, and means for varying said probing digital radio signals inresponse to said received digital data.
 13. The meteor burstcommunications system of claim 12 wherein said remote stations arespaced apart sufficiently that said digital data is transmitted fromeach remote station along a different reflection path to said masterstation.
 14. A meteor burst communications system comprising:at leasttwo spaced apart master stations, a plurality of groups of remotestations spaced at locations remote from said master stations and eachgroup associated with one of said master stations, each said masterstation including a radio transmitter for transmitting probing digitalradio signals having address portions for being directed from saidmaster station for reflection from meteor trails to said remote stationsassociated therewith, each of said remote stations including a radioreceiver for receiving said reflected probing digital radio signals fromsaid master stations, at least one sensor of physical characteristicsconnected with each said remote station, each remote station including atransmitter for transmitting digital data representative of the outputof said sensor to said master station via reflection from a meteortrail, and means for detecting when one master station receives aresponse from a remote station associated with another master stationand for notifying said another master station to eliminate therequirement of further probing for the received remote station.
 15. Themeteor burst communications system of claim 14 and further comprising:aportable test unit for conducting diagnostic tests on said remotestations and including means storing an address which responds toaddress portions of probing digital radio signals generated by each ofsaid master stations to enable communication between said portable testunit and any of said master stations.
 16. A meteor burst communicationssystem comprising:at least one master station, a plurality of remotestations spaced at locations remote from said master station, saidmaster station including a radio transmitter for transmitting probingdigital radio signals having an address portion of selectively variablelength in order to address any one or any group of said remote stations,said digital radio signals being directed from said master station forreflection from meteor trails to said remote stations, each of saidremote stations including a radio receiver for receiving said reflectedprobing digital radio signals from said master station, each of saidremote stations including address recognition circuitry having apredetermined digital address sequence stored therein and further havingmeans to compare the received address portions with said stored digitaladdress sequence, at least one sensor of physical characteristicsconnected with each said remote station, and each remote stationincluding a transmitter for transmitting digital data representative ofthe output of said sensor to said master station via reflection from ameteor trail if said received address portion compares with the mostsignificant bits of said stored digital address sequence.
 17. The meteorburst communications system of claim 16 wherein said sensor connected tosaid remote station comprises means for sensing the amount ofprecipitation.
 18. The meteor burst communications system of claim 16wherein said sensor comprises means for sensing the amount of snow inthe area of said remote location.
 19. The meteor burst communicationssystem of claim 16 wherein said sensor comprises means for sensing thetemperature.
 20. The meteor burst communications system of claim 16wherein said sensor generates analog signals, an analog-to-digitalconverter provided for converting said analog signals to digital signalsfor transmission from said remote station.
 21. The meteor burstcommunications system of claim 16 wherein said probing digital radiosignals comprise a plurality of sequentially transmitted data fields.22. The meteor burst communications system of claim 21 wherein one ofsaid data fields comprises a plurality of digital bits to determineproper bit phase.
 23. The meteor burst communications system of claim 21wherein one of said data fields comprises a plurality of synchronizationbits.
 24. The meteor burst communications system of claim 21 wherein oneof said data fields comprises said address portion which has a number ofbits dependent upon the number of remote stations desired to be probed.25. The meteor burst communications system of claim 24 wherein saidaddress portion includes bits which indicate the number of bits in theaddress.
 26. The meteor burst communications system of claim 16 andfurther comprising:at least one portable test unit for being transportedto each of said remote stations, means in said test unit for performingdiagnostic tests on said remote stations, and means in said test unitfor receiving and responding to predetermined address signalstransmitted by said master station.
 27. In a meteor burst communicationssystem having a master station and a plurality of remote stations, amaster station comprising:a transceiver having a transmitter portion; adigital computer for generating commands to control the transmitter andfor providing a variable bit probe to be transmitted to selected remotestations; and master station control logic for interfacing between saidcomputer and said transceiver, said control logic comprising:means forholding probe data bits from said computer and providing a serial bitstream to said transceiver; and means for holding a count of the totalnumber of data bits in the probe and providing an indication that thelast data bit has been provided to said transmitter.
 28. The masterstation of claim 27 wherein said control logic further comprises:meansfor generating a transmit interrupt request to said computer to acquirea new probe.
 29. The master station of claim 28 wherein said transmitinterrupt means in said control logic comprises:means for indicatingthat no probe is available to be entered in said probe data bit holdingmeans; and means for generating an interrupt code to identify theinterrupt source to said computer.
 30. The master station of claim 27wherein said control logic further comprises:means for keying on and offthe transmitter portion of said transceiver.
 31. The master station ofclaim 27 wherein said transceiver has a receiver portion comprising anumber of separate receiving channels, and wherein said control logicfurther comprises:means for each receiver channel for storing data bitsreceived in a transmission from one of the remote stations; means forindicating that received data is available for processing; and means forgenerating an interrupt code identifying the receiver channel to saidcomputer and providing an interrupt request to the computer.
 32. In ameteor burst communications system having a master station fortransmitting variable length probe signals to a plurality of remotestations, each remote station comprising:a transceiver having atransmitter portion and a receiver portion; said receiver portion havingmeans for detecting the variable length probe signals, data acquisitionmeans for obtaining data for transmission to the master station oversaid transmitter portion; and remote station control logic forinterfacing between said transceiver and said data acquisition means,said control logic comprising:means for defining the current state ofthe remote station; means for accepting probe data bits received anddetected by said receiver portion; means for providing a count of thenumber of bits in each field of a received probe, said means being resetat the start of a field and incrementing as each bit in the field isreceived; means for sequentially comparing station address code bitswith received probe address bits; and means for outputting data to saiddata acquisition means.
 33. The remote station of claim 32 wherein saidcontrol logic comprising:means for keying on and off the transmitterportion of said transceiver; means for supplying data from said dataacquisition means to the transmitter; and means for generating a checkcharacter to be inserted into the transmitted data stream.
 34. A methodof meteor burst communication, comprising the steps of:generating apolling sequence comprising a series of probes, each probe being aspecific binary code of a predetermined number of bits; transmittingeach probe in said polling sequence for a predetermined period of time;receiving said transmitted probes at a plurality of remote stations,each station having an identifying address; transmitting responsesignals from each of the remote stations polled by each probe code; andgenerating a new polling sequence to poll a group of specific remotes independence upon the identification of the remote stations responding,the probes in the new polling sequence being varied in the number ofbits in the probe code from the probes in the previous polling sequence.35. The method of claim 34 wherein the generation of a new pollingsequence involves:accumulating the identification of remote stationsthat have responded within a predetermined period of time; reportingdata received from the remote stations to be polled that have respondedwithin the predetermined time period; and generating a polling sequenceto poll the remaining remotes in the group.
 36. The method of claim 34wherein the period of time that each probe in the polling sequence istransmitted is dependent upon the number of probes in the pollingsequence.
 37. The method of claim 34 wherein the probe code in the newpolling sequence is reduced in the number of bits to increase the numberof remote stations polled.
 38. The method of claim 34 wherein the probecode in the new polling sequence is increased in the number of bits toreduce the number of remote stations polled.